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

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Date:   Mon, 19 Jul 2021 10:57:19 -0500
From:   Eddie James <eajames@...ux.ibm.com>
To:     David Laight <David.Laight@...LAB.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

On Sat, 2021-07-17 at 13:46 +0000, David Laight wrote:
> 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?

Probably so. However there is no choice in the matter, as the SPI
controller can't process larger reads/writes.
(I should note that the controller/driver CAN process up to 40 byte
writes. However, the driver must report the minimum of read and write
sizes in the max_transfer_size callback, so no client driver should
request larger than the max read size - 8 bytes).

Thanks,
Eddie

> 
> 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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