linux-kernel - Re: [PATCH v6 1/5] firmware: qcom_scm: provide a read-modify-write function

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Message-ID: <ZHEsELMsevxAR36x@surfacebook>
Date:   Sat, 27 May 2023 01:00:48 +0300
From:   andy.shevchenko@...il.com
To:     Mukesh Ojha <quic_mojha@...cinc.com>
Cc:     agross@...nel.org, andersson@...nel.org, konrad.dybcio@...aro.org,
        linus.walleij@...aro.org, linux-arm-msm@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-gpio@...r.kernel.org
Subject: Re: [PATCH v6 1/5] firmware: qcom_scm: provide a read-modify-write
 function

Wed, Mar 29, 2023 at 01:16:48PM +0530, Mukesh Ojha kirjoitti:
> It was realized by Srinivas K. that there is a need of
> read-modify-write scm exported function so that it can
> be used by multiple clients.
> 
> Let's introduce qcom_scm_io_update_field() which masks
> out the bits and write the passed value to that
> bit-offset. Subsequent patch will use this function.

...

> +	new = (old & ~mask) | val << (ffs(mask) - 1);

It's a bit non-standard to see left shift here instead of masking.

	new = (old & ~mask) | (val & mask);

is usual pattern.

Note as well that your code is prone to subtle mistakes when overflow may
easily override bits outside the mask.

> +	return qcom_scm_io_writel(addr, new);
> +}

-- 
With Best Regards,
Andy Shevchenko