Re: [PATCH][2/4] usb: only set URB_NO_TRANSFER_DMA_MAP when dma is enabled

On Sat, 26 Jan 2008, Magnus Damm wrote:

> Alan and David, thanks for your comments.
> 
> The HCD_LOCAL_MEM code only copies data into the bounce buffers if
> URB_NO_TRANSFER_DMA_MAP is unset. If that flag is set then the code
> expects the buffer to have been allocated using dma_alloc_coherent()
> and therefore there is no point in copying it. Or mapping it, as the
> original dma map code did. Anyway, so having that flag set even though
> dma is disabled and the dma handle invalid (data is not at all in
> buffers allocated using dma_alloc_coherent) breaks usb storage for
> ohci-sm501.

If usb-storage breaks then won't other drivers like ub break as well?  
The documentation for URB_NO_TRANSFER_DMA_MAP and URB_NO_SETUP_DMA_MAP
says only that the DMA handle must be valid; it doesn't say that the
buffer had to allocated by usb_buffer_alloc(), hcd_buffer_alloc(), or
dma_alloc_coherent().

Now, there's nothing wrong with your patch.  It avoids setting 
URB_NO_TRANSFER_DMA_MAP when the DMA handle is invalid.  But the way 
you are overloading the meaning of that flag could easily lead to 
problems with other drivers.

Alan Stern


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