linux-kernel - Re: [PATCH] mmc: core Drop BLK_BOUNCE

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Date: Sat, 10 Feb 2024 00:41:58 +0100
From: Linus Walleij <linus.walleij@...aro.org>
To: Michał Mirosław <mirq-linux@...e.qmqm.pl>
Cc: Arnd Bergmann <arnd@...db.de>, Christoph Hellwig <hch@....de>, Ulf Hansson <ulf.hansson@...aro.org>, 
	"linux-mmc @ vger . kernel . org" <linux-mmc@...r.kernel.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mmc: core Drop BLK_BOUNCE_HIGH

On Fri, Feb 9, 2024 at 11:35 PM Michał Mirosław <mirq-linux@...e.qmqm.pl> wrote:

> > I think it's worth mentioning the cb710 example here, which
> > uses a platform device as a child of a PCI device and
> > does not assign a DMA mask nor use DMA.
> >
> > This one will see a change in behavior, meaning that the
> > blockdev buffers are no longer bounced. As far as I can
> > tell, this is fine because the driver appears to correctly
> > use the sg_iter infrastructure for mapping data pages,
> > but it would be good to have this confirmed by
> > Michał Mirosław because this code path has probably never
> > been tested without BLK_BOUNCE_HIGH.
>
> Hi, this driver doesn't do DMA at all, so having DMA mask set or not
> it should be good as long as the CPU can read/write the buffers.

The only difference is where the CPU have to read/write the
buffers really, before the change those were all guaranteed to
be in lowmem (bounced there by the block core), now they can
also be in highmem, but sg_miter will deal with it for sure.

Yours,
Linus Wallej