linux-kernel - RE: [PATCH 1/2] spi: fsi: Reduce max transfer size to 8 bytes

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Date:   Tue, 20 Jul 2021 13:04:38 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Eddie James' <eajames@...ux.ibm.com>,
        Mark Brown <broonie@...nel.org>
CC:     "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "openbmc@...ts.ozlabs.org" <openbmc@...ts.ozlabs.org>,
        "robh+dt@...nel.org" <robh+dt@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-spi@...r.kernel.org" <linux-spi@...r.kernel.org>
Subject: RE: [PATCH 1/2] spi: fsi: Reduce max transfer size to 8 bytes

From: Eddie James <eajames@...ux.ibm.com>
> Sent: 19 July 2021 16:47
> To: Mark Brown <broonie@...nel.org>
> Cc: devicetree@...r.kernel.org; openbmc@...ts.ozlabs.org; robh+dt@...nel.org; linux-
> kernel@...r.kernel.org; linux-spi@...r.kernel.org
> Subject: Re: [PATCH 1/2] spi: fsi: Reduce max transfer size to 8 bytes
> 
> On Mon, 2021-07-19 at 16:20 +0100, Mark Brown wrote:
> > On Fri, Jul 16, 2021 at 01:34:38PM -0500, Eddie James wrote:
> >
> > > Security changes in the SPI controller - in the device microcode. I
> > > can
> > > reword the commit if you like.
> >
> > How will people end up running this device microcode?  Is this a bug
> > fix, or is this going to needlessly reduce performance for people
> > with
> > existing hardware?
> 
> The hardware is still in development. As part of the development, the
> device microcode was changed to restrict transfers. The reason for this
> restriction was "security concerns". This restriction disallows the
> loop (or branch-if-not-equal-and-increment) sequencer command. It also
> does not allow the read (or shift in if you prefer) command to specify
> the number of bytes in the command itself. Rather, the number of bits
> to shift in must be specified in a separate control register. This
> effectively means that the controller cannot transfer more than 8 bytes
> at a time.
> Therefore I suppose this is effectively a bug fix. There will be no
> hardware available without these restrictions, so it is not a needless
> reduction in performance. Every system that can run this driver will
> run the more restrictive device microcode.

So just say that release versions of the hardware won't support
more than 8 byte transfers.

Having said that, you might want a loop in the driver so that
application requests for longer transfers are implemented
with multiple hardware requests.

I do also wonder why there is support in the main kernel sources
for hardware that doesn't actually exist.

	David

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