linux-kernel - Re: [PATCH v2] pwm: bcm2835: Support apply function for atomic configuration

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <4683237c-7b40-11ab-b3c0-f94a5dd39b4d@gmx.de>
Date:   Fri, 4 Dec 2020 00:42:15 +0100
From:   Lino Sanfilippo <LinoSanfilippo@....de>
To:     Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
Cc:     thierry.reding@...il.com, lee.jones@...aro.org,
        nsaenzjulienne@...e.de, f.fainelli@...il.com, rjui@...adcom.com,
        sbranden@...adcom.com, bcm-kernel-feedback-list@...adcom.com,
        linux-pwm@...r.kernel.org, linux-rpi-kernel@...ts.infradead.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        sean@...s.org
Subject: Re: [PATCH v2] pwm: bcm2835: Support apply function for atomic
 configuration

Hi Uwe,

First off, thanks for the review!

On 29.11.20 at 19:10, Uwe Kleine-König wrote:

>
> Changelog between review rounds go to below the tripple-dash below.
>

Right, will do so in the next patch version.

>
> You're storing an unsigned long long (i.e. 64 bits) in an u32. If
> you are sure that this won't discard relevant bits, please explain
> this in a comment for the cursory reader.

What about an extra check then to make sure that the period has not been truncated,
e.g:

	value = DIV_ROUND_CLOSEST_ULL(state->period, scaler);

	/* dont accept a period that is too small or has been truncated */
	if ((value < PERIOD_MIN) ||
	    (value != DIV_ROUND_CLOSEST_ULL(state->period, scaler)))
		return -EINVAL;

> Also note that round_closed is probably wrong, as .apply() is
> supposed to round down the period to the next achievable period. (But
> fixing this has to do done in a separate patch.)

According to commit 11fc4edc4 rounding to the closest integer has been introduced
to improve precision in case that the pwm controller is used by the pwm-ir-tx driver.
I dont know how strong the requirement is to round down the period in apply(), but I
can imagine that this may be a good reason to deviate from this rule.
(CCing Sean Young who introduced DIV_ROUND_CLOSEST)

Regards,
Lino