Re: [PATCH v5 3/5] rpmsg: virtio_rpmsg_bus: get buffer size from config space

On 7/22/2026 9:49 AM, Mathieu Poirier wrote:
> On Tue, Jul 21, 2026 at 11:02:13AM -0500, Shah, Tanmay wrote:
>>
>>
>> On 7/21/2026 10:50 AM, Mathieu Poirier wrote:
>>> On Thu, Jul 16, 2026 at 11:12:55AM -0500, Shah, Tanmay wrote:
>>>>
>>>>
>>>> On 7/16/2026 10:48 AM, Mathieu Poirier wrote:
>>>>> On Wed, 15 Jul 2026 at 11:28, Shah, Tanmay <[email protected]> wrote:
>>>>>>
>>>>>> Hi,
>>>>>>
>>>>>> Please find my response below:
>>>>>>
>>>>>> On 7/15/2026 11:24 AM, Mathieu Poirier wrote:
>>>>>>> On Fri, Jul 10, 2026 at 12:28:29PM -0700, Tanmay Shah wrote:
>>>>>>>> 512 bytes isn't always suitable for all case, let firmware
>>>>>>>> maker decide the best value from resource table.
>>>>>>>> enable by VIRTIO_RPMSG_F_BUFSZ feature bit.
>>>>>>>>
>>>>>>>> Signed-off-by: Tanmay Shah <[email protected]>
>>>>>>>> ---
>>>>>>>> Changes in v5:
>>>>>>>>
>>>>>>>>   - fix documentation about alignment of the buffer size
>>>>>>>>   - change version field from u16 to u8
>>>>>>>>   - remove buffer alignment check
>>>>>>>>   - Separate buffer alignment vs MTU of a single buffer
>>>>>>>>   - Use buffer alignment only to get next buffer address at alignment
>>>>>>>>     boundary
>>>>>>>>
>>>>>>
>>>>>> [...]
>>>>>>
>>>>>>>> +#ifndef _LINUX_VIRTIO_RPMSG_H
>>>>>>>> +#define _LINUX_VIRTIO_RPMSG_H
>>>>>>>> +
>>>>>>>> +#include <linux/types.h>
>>>>>>>> +#include <linux/virtio_types.h>
>>>>>>>> +
>>>>>>>> +/* The feature bitmap for virtio rpmsg */
>>>>>>>> +#define VIRTIO_RPMSG_F_NS   0 /* RP supports name service notifications */
>>>>>>>> +#define VIRTIO_RPMSG_F_BUFSZ        1 /* RP get buffer size from config space */
>>>>>>>> +
>>>>>>>> +/* Version of struct virtio_rpmsg_config understood by this driver */
>>>>>>>> +#define RPMSG_VDEV_CONFIG_V1        1
>>>>>>>> +
>>>>>>>> +/**
>>>>>>>> + * struct virtio_rpmsg_config - config space for rpmsg virtio device
>>>>>>>> + *
>>>>>>>> + * @version:        version of this structure, currently %RPMSG_VDEV_CONFIG_V1.
>>>>>>>> + * @size:   size of this structure in bytes.
>>>>>>>> + * @rpmsg_buf_align: alignment in bytes for each buffer. Must be a power of
>>>>>>>> + *               two. If 0 then no alignment will be done. This alignment
>>>>>>>> + *               will not decide actual size of the buffer but will be
>>>>>>>> + *               used to decided the start address of the buffer. The
>>>>>>>> + *               actual size of the buffer can be different than the
>>>>>>>> + *               aligned size of the buffer.
>>>>>>>
>>>>>>> Is there really a need to have a buffer size different from its alignment?  It's
>>>>>>> not like the (small) delta between the buffer size and its alignment will be
>>>>>>> used for something else.  I'm fine with a buffer alignment requirement but in
>>>>>>> those cases, the firmware should set the size of the buffer in accordance with
>>>>>>> its alignment requirement.  Otherwise, the complexity needed to manage the
>>>>>>> discrpancy between the two yields a driver that is hard to maintain and prone to
>>>>>>> bugs.
>>>>>>>
>>>>>>
>>>>>> I had the same concern before. However, following example changed my mind:
>>>>>>
>>>>>> So, a single buffer size is the MTU size of a packet for the protocol
>>>>>> supported by the firmware. Now that can be different than the aligned
>>>>>> size of the buffer.
>>>>>>
>>>>>> For example, the higher level protocol (not rpmsg) has 430 bytes as the
>>>>>> max size of a payload. However, cache line alignment is 64-bytes. Then
>>>>>> in that case, the aligned buffer size is 448 bytes. But, that doesn't
>>>>>> mean we can say protocol's MTU size is 448 bytes. If user end up
>>>>>> treating MTU size 448 bytes and use space beyond 430 bytes, then the
>>>>>> higher level apps might discard that data and communication may fail.
>>>>>>
>>>>>
>>>>> How is that scenario different from today's 512 byte buffer size?
>>>>> Most users don't use all 512 bytes and we don't run in the problem
>>>>> described above?
>>>>>
>>>>
>>>> 512 buffer size is hardcoded, so it is enforced on the protocol by the
>>>> framework. But by allowing the configuration of the buffer size we are
>>>> allowing the protocol to decide what the buffer size should be. So, when
>>>> user request the buffer via rpmsg_get_mtu() API, then that should be the
>>>> original buffer size which is expected by the protocol, which may not be
>>>> same as the aligned buffer size.
>>>
>>> Regardless of the buffer size, whether it is set to 512 byte or some arbitrary
>>> value by the remote processsor's firmware, there is a possibility of a
>>> discrepancy with what is expected by the protocol.  Right now rpmsg_get_mtu()
>>> returns 512 regardless of what a protocol uses.  The only thing that should be
>>> important to the protocol is not to exceed that limit.
>>>
>>
>> I think I am missing something. Are you saying that buffer size can not
>> be configured greater than 512 bytes?
> 
> I am not.
> 
> What I am saying is that if alignment is important to a remote processor, it
> should choose the buffer size accordingly.  rpmsg_get_mtu() should return the
> value of the buffer size, exactly the way it is today. 
> 
>>
>> If the higher level protocol (not RPMsg) wants to use 4030 bytes for
>> single packet payload then that is what the MTU size should be. And so
>> the firmware will configure 4030 bytes as single buffer size in the vdev
>> config space. That is why alignment should be treated separately.
>> Because it is not equal to payload size needed by higher level protocol.
> 
> In that case and assuming alignment is required, the buffer size should be 4096
> and rpmsg_get_mtu() should also return 4096.  How a higher protocol uses the
> buffer space is none of our concern.
> 
> Currently, the buffer size is set to 512 and users don't always fill the entire
> buffer.  I don't see why things should be different with a configurable buffer
> size.   
> 
>>
>>>>
>>>> If for internal management we want to treat buffer size = aligned buffer
>>>> size, I am okay. But rpmsg_get_mtu() must give unaligned buffer size
>>>> which is expected by the protocol.
>>>
>>> I agree with the first sentence but not the second.  The only thing protocols
>>> should care about is the start address of a buffer and that its size is
>>> sufficient for what it needs.
>>>

Ack. I am convinced. If we don't see any functional failure then no
point in keeping the complexity. At best, some space will be unused but
that should be okay.

Thanks.

>>>>
>>>> Thanks,
>>>> Tanmay
>>>>
>>>>>> The alignment field is used only to decide where the next buffer start
>>>>>> address is to ease cache operations.
>>>>>>
>>>>>> Sure, we need to maintain this complexity, but I think it's worth it.
>>>>>>
>>>>>
>>>>> The same as in my previous email to Arnaud applies here - is this an
>>>>> immediate requirement of something we think may be happening in the
>>>>> future?
>>>>>
>>>>
>>>> IMHO, vendors will use it if the feature is available, otherwise the
>>>> need to optimize alignment is not easily encountered.
>>>>
>>>>>
>>>>>> Thanks,
>>>>>> Tanmay
>>>>>>
>>>>>>>> + * @txbuf_size:     Tx buf size from remote's view. For Linux this is rx buf size.
>>>>>>>> + * @rxbuf_size:     Rx buf size from remote's view. For Linux this is tx buf size.
>>>>>>>> + *
>>>>>>>> + * This is the configuration structure shared by the device and the driver,
>>>>>>>> + * read when %VIRTIO_RPMSG_F_BUFSZ is negotiated. The fields are laid out so
>>>>>>>> + * the structure is naturally 32-bit aligned.
>>>>>>>> + */
>>>>>>>> +struct virtio_rpmsg_config {
>>>>>>>> +    u8 version;
>>>>>>>> +    __virtio16 size;
>>>>>>>> +    __virtio16 rpmsg_buf_align;
>>>>>>>> +    /* The tx/rx individual buffer size (if VIRTIO_RPMSG_F_BUFSZ) */
>>>>>>>> +    __virtio32 txbuf_size;
>>>>>>>> +    __virtio32 rxbuf_size;
>>>>>>>> +} __packed;
>>>>>>>> +
>>>>>>>> +#endif /* _LINUX_VIRTIO_RPMSG_H */
>>>>>>>> --
>>>>>>>> 2.34.1
>>>>>>>>
>>>>>>
>>>>
>>