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[217.229.27.143]) by smtp.gmail.com with ESMTPSA id o4-v6sm28897042wra.3.2018.07.09.09.57.25 (version=TLS1_2 cipher=ECDHE-RSA-CHACHA20-POLY1305 bits=256/256); Mon, 09 Jul 2018 09:57:25 -0700 (PDT) Date: Mon, 9 Jul 2018 18:57:24 +0200 From: Thierry Reding To: Thomas Hebb Cc: linux-kernel@vger.kernel.org, Antoine Tenart , sebastian.hesselbarth@gmail.com, Jisheng.Zhang@synaptics.com, "open list:PWM SUBSYSTEM" Subject: Re: [PATCH v2] pwm: berlin: Don't use broken prescaler values Message-ID: <20180709165724.GB26651@ulmo> References: MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="St7VIuEGZ6dlpu13" Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.10.0 (2018-05-17) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org --St7VIuEGZ6dlpu13 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Wed, Jun 06, 2018 at 01:42:10PM -0400, Thomas Hebb wrote: > The Berlin PWM driver is currently broken on at least BG2CD. The > symptoms manifest as a very non-linear and erratic mapping from the duty > cycle configured in software to the duty cycle produced by hardware. >=20 > The cause of the bug is software's configuration of the prescaler, and > in particular its usage of the six prescaler values between the minimum > value of 1 and the maximum value of 4096. As it turns out, these six > values do not actually slow down the PWM clock; rather, they emulate > slowing down the clock by internally multiplying the value of TCNT. >=20 > This would be a fine trick, if not for the fact that the internal, > scaled TCNT value has no extra bits beyond the 16 already exposed to > software in the register. What this means is that, for a prescaler of 4, > the software must ensure that the top two bits of TCNT are not set, > because hardware will chop them off; for a prescaler of 8, the top three > bits must not be set, and so forth. Software does not currently ensure > this, resulting in a TCNT several orders of magnitude lower than > intended any time one of those six prescalers are selected. >=20 > Because hardware chops off the high bits in its internal shift, the > middle six prescalers don't actually allow *anything* that the first > doesn't. In fact, they are strictly worse than the first, since the > internal shift of TCNT prevents software from setting the low bits, > decreasing the resolution, without providing any extra high bits. >=20 > By skipping the useless prescalers entirely, this patch both fixes the > driver's behavior and increases its performance (since, when the 4096 > prescaler is selected, it now does only a single shift rather than the > seven successive divisions it did before). >=20 > Tested on BG2CD. >=20 > Signed-off-by: Thomas Hebb > --- > drivers/pwm/pwm-berlin.c | 45 ++++++++++++++++++++++------------------ > 1 file changed, 25 insertions(+), 20 deletions(-) Applied, thanks. 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