linux-kernel - Re: [PATCH v11 03/10] spi: Add multi-cs memories support in SPI core

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Date: Sun, 21 Jan 2024 11:29:48 -0800
From: Guenter Roeck <linux@...ck-us.net>
To: Michael Walle <michael@...le.cc>
Cc: Mark Brown <broonie@...nel.org>,
 Amit Kumar Mahapatra <amit.kumar-mahapatra@....com>,
 tudor.ambarus@...aro.org, pratyush@...nel.org, miquel.raynal@...tlin.com,
 richard@....at, vigneshr@...com, sbinding@...nsource.cirrus.com,
 lee@...nel.org, james.schulman@...rus.com, david.rhodes@...rus.com,
 rf@...nsource.cirrus.com, perex@...ex.cz, tiwai@...e.com,
 linux-spi@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-mtd@...ts.infradead.org, nicolas.ferre@...rochip.com,
 alexandre.belloni@...tlin.com, claudiu.beznea@...on.dev,
 michal.simek@....com, linux-arm-kernel@...ts.infradead.org,
 alsa-devel@...a-project.org, patches@...nsource.cirrus.com,
 linux-sound@...r.kernel.org, git@....com, amitrkcian2002@...il.com
Subject: Re: [PATCH v11 03/10] spi: Add multi-cs memories support in SPI core

On 1/21/24 10:06, Michael Walle wrote:
>>>> FWIW, the problem is due to
>>>
>>>> +#define SPI_CS_CNT_MAX 4
>>>
>>>> in the offending patch, but apeed2400 FMC supports up to 5 SPI chip selects.
>>>>
>>>>   static const struct aspeed_spi_data ast2400_fmc_data = {
>>>>          .max_cs        = 5,
>>>>     ^^^^^^^^^^^^^^^^^^^
>>>>          .hastype       = true,
>>>
>>>> Limiting .max_cs to 4 or increasing SPI_CS_CNT_MAX to 5 fixes the problem,
>>>> though of course I don't know if increasing SPI_CS_CNT_MAX has other side
>>>> effects.
>>>
>>> Yeah, I was coming to a similar conclusion myself - the limit is just
>>> too low.  I can't see any problem with increasing it.
>>
>> It would cost a bit of memory and somewhat affect performance sine many
>> of the newly introduced loops are bound by SPI_CS_CNT_MAX and not by
>> num_chipselect.
>>
>> It also might make sense to document the new limit somewhere. Prior
>> to this commit it was not limited at all.
>> Documentation/devicetree/bindings/spi/spi-davinci.txt lists 5 chip
>> selects in its example for the use of cs-gpios.
>> Documentation/devicetree/bindings/spi/spi-controller.yaml also does not
>> list a limit.
> 
> Given that, that the rest of this series is under discussion (and esp. whether
> it is the correct way to do it) it might make sense to just revert the picked
> patches.
> 

I can't really comment on that, but I found that there is another
affected devicetree property: num-cs. Its range isn't limited in
Documentation/devicetree/bindings/spi/spi-controller.yaml. Various
dts/dtsi files use numbers as large as 8. The range is limited in some
bindings files, but not all of them.
Documentation/devicetree/bindings/spi/spi-lantiq-ssc.txt, for example,
says

   "num-cs: see spi-bus.txt, set to 8 if unset"

Various Broadcom dtsi files set it to 8.

So I guess 8 would be the absolute minimum to re-enable support for
affected systems.

Guenter