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Date: Sat, 10 Feb 2024 12:58:30 +0100
From: "Arnd Bergmann" <arnd@arndb.de>
To: "Linus Walleij" <linus.walleij@linaro.org>,
 =?UTF-8?Q?Micha=C5=82_Miros=C5=82aw?= <mirq-linux@rere.qmqm.pl>
Cc: "Christoph Hellwig" <hch@lst.de>, "Ulf Hansson" <ulf.hansson@linaro.org>,
 "linux-mmc @ vger . kernel . org" <linux-mmc@vger.kernel.org>,
 linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mmc: core Drop BLK_BOUNCE_HIGH
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On Sat, Feb 10, 2024, at 00:41, Linus Walleij wrote:
> On Fri, Feb 9, 2024 at 11:35=E2=80=AFPM Micha=C5=82 Miros=C5=82aw <mir=
q-linux@rere.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
>> > Micha=C5=82 Miros=C5=82aw 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.

Yes, that was my point: The sg_miter() code is meant to
handle exactly this case with highmem data, but as far
as I can tell, that code path has never been tested on
32-bit systems with highmem but without BLK_BOUNCE_HIGH.

     Arnd