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

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Date: Sat, 10 Feb 2024 20:38:27 +0100
From: Linus Walleij <linus.walleij@...aro.org>
To: Arnd Bergmann <arnd@...db.de>
Cc: Michał Mirosław <mirq-linux@...e.qmqm.pl>, 
	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 Sat, Feb 10, 2024 at 12:58 PM Arnd Bergmann <arnd@...db.de> wrote:
> On Sat, Feb 10, 2024, at 00:41, Linus Walleij wrote:

> > 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.

It's actually possible to enforce testing of highmem scatterlists
to an MMC card (one need to be careful as this is destructive
testing!)
drivers/mmc/core/mmc_test.c

..but the one relevant target I have is a Kirkwood and it only
has 128 MB of memory so highmem won't be exercised.

I'll put this into the cover letter on the other series (fixing a bunch
of drivers to use sg_miter) though.

Yours,
Linus Walleij