Re: Buffer management in the zr36067 driver

Hi,

On 6/9/07, Jean Delvare <[email protected]> wrote:
>
> The xfs code uses a 128 kB limit, too. From LDD3, about kmalloc:
> "If your code is to be completely portable, it cannot count on being
> able to allocate anything larger than 128 KB."


_If hardcoded_.

The maximum order seems to be 10 on x86_64. This is 4 MB. Even if we
> can't get that, being able to get 512 kB or 1 MB would be a significant
> improvement over the current situation. Maybe we can even get 2 or 4 MB
> if we preallocate the buffers on driver load. Arguably, this can be
> seen as a waste of kernel memory, so maybe this should be an option
> rather than the default.

[..]

> I believe that the driver should not hard-code a memory limit value.
> Instead, it should always try kmalloc (or __get_free_pages), and it
> should fallback to other methods (get_high_mem, bigphys_area) only if
> kmalloc fails. What do you think?
>
[..]

> Without CONFIG_BIGPHYS_AREA (default), the driver uses get_high_mem()
> to allocate large buffers, which looks like a gross, totally unsafe
> hack to me. And it doesn't work for me at least, the largest amount I
> manage to get is 32 kB. Does this really work for anyone? I'm surprised
> it was even accepted into the kernel tree. Shouldn't we remove this
> from the driver?


All yes.

Other v4l drivers (bttv, cx88) use a helper module called video-buf.
> The header comment says it implements non-contiguous, PCI-DMA-able
> buffers for video drivers. Couldn't this be used in the zr36067 driver
> as well?


No, it's non-contiguous, and thus useless for us. The whole base design is
around that, unfortunately.

Ronald

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