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From: David Laight <David.Laight@ACULAB.COM>
To: 'Eddie James' <eajames@linux.ibm.com>, Mark Brown <broonie@kernel.org>
Cc: "devicetree@vger.kernel.org" <devicetree@vger.kernel.org>,
	"openbmc@lists.ozlabs.org" <openbmc@lists.ozlabs.org>,
	"robh+dt@kernel.org" <robh+dt@kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"linux-spi@vger.kernel.org" <linux-spi@vger.kernel.org>
Subject: RE: [PATCH 1/2] spi: fsi: Reduce max transfer size to 8 bytes
Date: Tue, 20 Jul 2021 13:04:38 +0000	[thread overview]
Message-ID: <0a637d7704df4303abe783215080578d@AcuMS.aculab.com> (raw)
In-Reply-To: <d2e07f0beda57ffeaa31e8cf5bf28edfbd982e58.camel@linux.ibm.com>

From: Eddie James <eajames@linux.ibm.com>
> Sent: 19 July 2021 16:47
> To: Mark Brown <broonie@kernel.org>
> Cc: devicetree@vger.kernel.org; openbmc@lists.ozlabs.org; robh+dt@kernel.org; linux-
> kernel@vger.kernel.org; linux-spi@vger.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

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)

  reply	other threads:[~2021-07-20 13:06 UTC|newest]

Thread overview: 14+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-07-16 13:39 [PATCH 0/2] spi: fsi: Reduce max transfer size to 8 bytes Eddie James
2021-07-16 13:39 ` [PATCH 1/2] " Eddie James
2021-07-16 17:19   ` Mark Brown
2021-07-16 18:34     ` Eddie James
2021-07-19 15:20       ` Mark Brown
2021-07-19 15:46         ` Eddie James
2021-07-20 13:04           ` David Laight [this message]
2021-07-20 13:13             ` Mark Brown
2021-07-20 17:32               ` David Laight
2021-07-16 13:39 ` [PATCH 2/2] dt-bindings: fsi: Remove ibm, fsi2spi-restricted compatible Eddie James
2021-07-19 15:54   ` [PATCH 2/2] dt-bindings: fsi: Remove ibm,fsi2spi-restricted compatible Mark Brown
2021-07-17 13:46 ` [PATCH 0/2] spi: fsi: Reduce max transfer size to 8 bytes David Laight
2021-07-19 15:57   ` Eddie James
2021-07-19 18:00 ` Mark Brown

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