linux-kernel - RE: [PATCH 0/2] spi: fsi: Reduce max transfer size to 8 bytes

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Message-ID: <0ed7c393b7aa4252a48cd2397ac251eb@AcuMS.aculab.com>
Date:   Sat, 17 Jul 2021 13:46:15 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Eddie James' <eajames@...ux.ibm.com>,
        "linux-spi@...r.kernel.org" <linux-spi@...r.kernel.org>
CC:     "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "robh+dt@...nel.org" <robh+dt@...nel.org>,
        "broonie@...nel.org" <broonie@...nel.org>,
        "openbmc@...ts.ozlabs.org" <openbmc@...ts.ozlabs.org>,
        "joel@....id.au" <joel@....id.au>
Subject: RE: [PATCH 0/2] spi: fsi: Reduce max transfer size to 8 bytes

From: Eddie James
> Sent: 16 July 2021 14:39
> 
> The security restrictions on the FSI-attached SPI controllers have
> been applied universally to all controllers, so the controller can no
> longer transfer more than 8 bytes for one transfer. Refactor the driver
> to remove the looping and support for larger transfers, and remove the
> "restricted" compatible string, as all the controllers are now
> considered restricted.

Aren't there significant performance (and device wear?) penalties
for doing short SPI eeprom writes?

IIRC (and I'm not getting my serial busses confused) a write request
can request an aligned transfer of up to (typically) 32 bytes.
At which point you need to wait for the status to indicate 'complete'.

So restricting writes to 8 bytes increases block write times
by a factor of 4.

Increasing the numbers of writes by a factor or 4 may also have
an effect on device wear - but that is more likely only affected
by erase cycles.

	David

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