linux-kernel - Re: [PATCH v2] regmap-i2c: Subtract reg size from max

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Message-ID: <ZlwfQ3ERbo5snLcK@surfacebook.localdomain>
Date: Sun, 2 Jun 2024 10:29:07 +0300
From: Andy Shevchenko <andy.shevchenko@...il.com>
To: Jim Wylder <jwylder@...gle.com>
Cc: Mark Brown <broonie@...nel.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	"Rafael J. Wysocki" <rafael@...nel.org>, kernel-team@...roid.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] regmap-i2c: Subtract reg size from max_write

Thu, May 23, 2024 at 04:14:36PM -0500, Jim Wylder kirjoitti:
> Currently, when an adapter defines a max_write_len quirk,
> the data will be chunked into data sizes equal to the
> max_write_len quirk value.  But the payload will be increased by
> the size of the register address before transmission.  The
> resulting value always ends up larger than the limit set
> by the quirk.
> 
> Avoid this error by setting regmap's max_write to the quirk's
> max_write_len minus the number of bytes for the register and
> padding.  This allows the chunking to work correctly for this
> limited case without impacting other use-cases.

...

>  		    (bus->max_raw_write == 0 || bus->max_raw_write > quirks->max_write_len))
> -			max_write = quirks->max_write_len;
> +			max_write = quirks->max_write_len -
> +				(config->reg_bits + config->pad_bits) / BITS_PER_BYTE;

Interesting. regmap-spi does this slightly differently, i.e. reg/8 + pad/8.
I'm wondering which one is more correct (potential off-by-one I suppose).

-- 
With Best Regards,
Andy Shevchenko