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[217.229.16.64]) by smtp.gmail.com with ESMTPSA id b129sm1235602wmd.24.2019.01.29.02.15.06 (version=TLS1_2 cipher=ECDHE-RSA-CHACHA20-POLY1305 bits=256/256); Tue, 29 Jan 2019 02:15:06 -0800 (PST) Date: Tue, 29 Jan 2019 11:15:05 +0100 From: Thierry Reding To: Dmitry Osipenko Cc: Sowjanya Komatineni , Jonathan Hunter , Mantravadi Karthik , Shardar Mohammed , Timo Alho , "linux-tegra@vger.kernel.org" , "linux-kernel@vger.kernel.org" , "linux-i2c@vger.kernel.org" Subject: Re: [PATCH V3 2/3] i2c: tegra: Update transfer timeout Message-ID: <20190129101505.GA28850@ulmo> References: <1548475073-12408-1-git-send-email-skomatineni@nvidia.com> <1548475073-12408-2-git-send-email-skomatineni@nvidia.com> <0cf91475-f77d-7453-deb6-3dd91b63aeb6@gmail.com> <2f3cab66-7424-1b33-976f-826877623726@gmail.com> MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="VbJkn9YxBvnuCH5J" Content-Disposition: inline In-Reply-To: <2f3cab66-7424-1b33-976f-826877623726@gmail.com> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org --VbJkn9YxBvnuCH5J Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Tue, Jan 29, 2019 at 01:27:09AM +0300, Dmitry Osipenko wrote: > 29.01.2019 1:15, Sowjanya Komatineni =D0=BF=D0=B8=D1=88=D0=B5=D1=82: > >=20 > >=20 > >>>>> Update I2C transfer timeout based on transfer bytes and I2C bus rat= e=20 > >>>>> to allow enough time during max transfer size based on the speed. > >>>> > >>>> Could it be that I2C device is busy and just slowly handling the tra= nsfer requests? Maybe better to leave the timeout as-is and assume the wors= t case scenario? > >>>> > >>> This change includes min transfer time out of 100ms in addition to co= mputed timeout based on transfer bytes and speed which can account in cases= of slave devices running at slower speed. > >>> Also Tegra I2C Master supports Clock stretching by the slave. > >> > >> Okay, I suppose in reality this shouldn't break anything. > >> > >> Please explain what benefits this change brings. Does it fix or improv= e anything? The commit message only describes changes done in the patch and= has no word on justification of those changes. Transfer timeout is an extr= eme case that doesn't happen often and > > when it happens, usually only th= e fact of timeout matters. If there is no real value in shortening of the t= imeout, why bother then? > >=20 > > Original transfer timeout in existing driver is 1Sec and incases of tra= nsfer size more than 10Kbytes at STD bus rate, timeout is not sufficient. > > Also Tegra194 platform supports max of 64K bytes of transfer and to all= ow full transfer size at lowest bus rate it takes almost ~5.8 Sec. > > In cases if large transfer at low bus rates 1 Sec timeout is not enough= and in those cases transfers will timeout before it waits for complete tra= nsfer to happen. > >=20 > > So this patch uses transfer time based on transfer bytes and bus rate. > >=20 >=20 > Please add that to the commit message. >=20 > And then seems you also need to set I2C adapter timeout to a some > larger value. Currently Tegra's I2C doesn't explicitly specify the > "adapter.timeout" and I2C core sets it to 1 second if it is 0. I don't think adapter.timeout is the same thing as the timeout that we're dealing with here. adapter.timeout is only used as a way to retry on arbitration lost conditions. So, as far as I understand, if we try to send a message to an I2C slave and we loose arbitration, we'll keep retrying for one second before finally failing. I wouldn't expect these arbitration lost conditions to happen right in the middle of a transfer, but rather at the beginning already, so I'm not sure arbitration could even be lost after, say, 2 seconds into a very large transfer. All of that said, it seems like i2c-tegra doesn't respect the protocol for making this arbitration loss retry work in the first place. We should be returning -EAGAIN if we detect arbitration loss, but I don't see that we ever return that. We should probably fix that in another patch. 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