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From:   Jonas Gorski <jonas.gorski@gmail.com>
Date:   Tue, 6 Mar 2018 16:33:34 +0100
Message-ID: <CAOiHx=n6yEB8Yq6rDjYKdfos0Ha6=8uLY1H-cwduw62d4nWTtQ@mail.gmail.com>
Subject: Re: [PATCH AUTOSEL for 4.9 002/219] spi/bcm63xx: make spi subsystem
 aware of message size limits
To:     Mark Brown <broonie@kernel.org>
Cc:     Sasha Levin <Alexander.Levin@microsoft.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "stable@vger.kernel.org" <stable@vger.kernel.org>
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On 6 March 2018 at 15:20, Mark Brown <broonie@kernel.org> wrote:
> On Tue, Mar 06, 2018 at 02:42:43PM +0100, Jonas Gorski wrote:
>> On 5 March 2018 at 21:35, Mark Brown <broonie@kernel.org> wrote:
>
>> > It's exposing more capability information but it's in the "how did this
>> > ever work without the fix" range, and I'd worry that this might cause us
>> > to do something like start exercising handling code in client drivers
>> > that had never been tested.  Not that I can immediately see any client
>> > drivers in mainline that actually pay attention...  :/
>
>> I would assume that most spi client drivers use short messages, so
>> they aren't affected unless the max message size is really short.
>> m25p80 on the other hand will do arbitrarily large transfers/reads, so
>> there it was supported first.
>
> There's a bunch of SPI drivers that do firmware downloads which I'd
> expect to be affected, the limit the device has is tiny so it's
> relatively easy to bump into it.  It's very rare for devices to be so
> limited so unfortunately client drivers don't generally check though.

Well, at least for bcm63xx it's very rare to have something other than
a flash chip, a (broadcom) ethernet switch management interface, or a
SLIC/SLAC attached to the SPI controller. And AFAICT of these three
only the flash chip uses large SPI transfers. Furthermore, unless you
have a development board, you won't be able to attach anything
different to it. So the chance to bump into the limits with other
drivers is rather low.

I would assume that this is true for most systems with a limited SPI
controller. I would hope that most board designers are sensible enough
to not add devices that won't work ;-)


Regards
Jonas