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Sat, 10 Feb 2024 06:58:51 -0500 (EST) X-Mailer: MessagingEngine.com Webmail Interface User-Agent: Cyrus-JMAP/3.11.0-alpha0-144-ge5821d614e-fm-20240125.002-ge5821d61 Precedence: bulk X-Mailing-List: linux-mmc@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Message-Id: <87e339bf-6ca9-406a-9f15-d744f90c9c40@app.fastmail.com> In-Reply-To: References: <20240125-mmc-no-blk-bounce-high-v1-1-d0f92a30e085@linaro.org> <8ca6a46e-551a-4400-965f-f4ad60bff072@app.fastmail.com> Date: Sat, 10 Feb 2024 12:58:30 +0100 From: "Arnd Bergmann" To: "Linus Walleij" , =?UTF-8?Q?Micha=C5=82_Miros=C5=82aw?= Cc: "Christoph Hellwig" , "Ulf Hansson" , "linux-mmc @ vger . kernel . org" , linux-kernel@vger.kernel.org Subject: Re: [PATCH] mmc: core Drop BLK_BOUNCE_HIGH Content-Type: text/plain;charset=utf-8 Content-Transfer-Encoding: quoted-printable 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 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