Re: [PATCH 2/2] media: usb: pwc: Don't use coherent DMA buffers for ISO transfer

Mauro Carvalho Chehab
2018-07-30T17:43:06Z

Em Tue, 17 Jul 2018 17:10:22 -0300
Ezequiel Garcia <ezequiel@collabora.com> escreveu:

> Yeah, and not setting URB_NO_TRANSFER_DMA_MAP makes the USB core
> create DMA mappings and use the streaming API. Which makes more
> sense in hardware without hardware coherency.
>
> The only thing that bothers me with this patch is that it's not
> really something specific to this driver. If this fix is valid
> for pwc, then it's valid for all the drivers allocating coherent
> memory.

We're actually doing this change on other drivers:
https://git.linuxtv.org/media_tree.git/commit/?id=d571b592c6206

I suspect that the reason why all USB media drivers were using
URB_NO_TRANSFER_DMA_MAP is just because the first media USB driver
upstream used it.

On that time, I remember I tried once to not use this flag, but there
was something that broke (perhaps I just didn't know enough about the
USB layer - or perhaps some fixes happened at USB core - allowing it
to be used with ISOC transfers).

Anyway, nowadays, I fail to see a reason why not let the USB core
do the DMA maps. On my tests after this patch, at the boards I tested
(arm and x86), I was unable to see any regressions.

Thanks,
Mauro