linux-kernel - Re: [PATCH] spi: sc18is602: Don't be that restrictive with the maximum transfer speed

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Message-ID: <5326F7D1.2050800@roeck-us.net>
Date:	Mon, 17 Mar 2014 06:25:37 -0700
From:	Guenter Roeck <linux@...ck-us.net>
To:	Mark Brown <broonie@...nel.org>
CC:	linux-spi@...r.kernel.org, linux-kernel@...r.kernel.org,
	Axel Lin <axel.lin@...ics.com>
Subject: Re: [PATCH] spi: sc18is602: Don't be that restrictive with the maximum
 transfer speed

On 03/17/2014 04:52 AM, Mark Brown wrote:
> On Sun, Mar 16, 2014 at 06:47:50PM -0700, Guenter Roeck wrote:
>> Commit 09e99bca8 (spi: sc18is602: Convert to let spi core validate
>> transfer speed) made the maximum transfer speed much more restrictive
>> than before. The transfer speed used to be adjusted to 1/4 of the chip
>> clock rate if a higher transfer speed was requested. Now such transfers are
>> simply rejected. With default settings, this causes, for example, a transfer
>> request at 2 mbps to be rejected because the maximum speed with the default
>> chip clock is 1.843 mbps.
>
>> This is unnecessarily restrictive and causes unnecessary failures. Loosen
>> the limit to accept transfers up to 50% of the clock rate and adjust
>> the speed as needed when setting up the actualt transfer.
>
> Given this description I'd expect to see a change in the core not a
> driver - like the other fork of the thread said I'd expect to deal with
> the issue by improving the constraint handling code.
>
Agreed.

>>   	master->transfer_one_message = sc18is602_transfer_one;
>>   	master->dev.of_node = np;
>>   	master->min_speed_hz = hw->freq / 128;
>> -	master->max_speed_hz = hw->freq / 4;
>> +	master->max_speed_hz = hw->freq / 2;
>
> That said, if this is something that the hardware can support it makes
> sense to do it anyway - is there an actual spec constraint available?
>

No, the technical fastest transfer speed is hz / 4, so this would be just an
arbitrary limit to be less restrictive. Axel's patch for the spi core works
perfectly, so I would suggest to go with it if that is acceptable for you.

Thanks,
Guenter

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