From mboxrd@z Thu Jan 1 00:00:00 1970 From: Mark Brown Subject: Re: [PATCH] RFC: mmc: core: Increase delay for voltage to stabilize from 3.3V to 1.8V Date: Wed, 13 May 2015 12:09:15 +0100 Message-ID: <20150513110915.GG3066@sirena.org.uk> References: <1431467171-12937-1-git-send-email-dianders@chromium.org> Mime-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="k040vn1t/h12DMPO" Return-path: Received: from mezzanine.sirena.org.uk ([106.187.55.193]:37005 "EHLO mezzanine.sirena.org.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S933242AbbEMMQY (ORCPT ); Wed, 13 May 2015 08:16:24 -0400 Content-Disposition: inline In-Reply-To: <1431467171-12937-1-git-send-email-dianders@chromium.org> Sender: linux-mmc-owner@vger.kernel.org List-Id: linux-mmc@vger.kernel.org To: Doug Anderson Cc: Ulf Hansson , linux-mmc@vger.kernel.org, linux-arm-kernel@lists.infradead.org, johan.rudholm@axis.com, adrian.hunter@intel.com, tim.kryger@gmail.com, javier.martinez@collabora.co.uk, andrew_gabbasov@mentor.com, s.hauer@pengutronix.de, linux-kernel@vger.kernel.org --k040vn1t/h12DMPO Content-Type: text/plain; charset=us-ascii Content-Disposition: inline On Tue, May 12, 2015 at 02:46:11PM -0700, Doug Anderson wrote: > Since the regulator used for the SDMMC IO voltage is not expected to > draw a lot of current, most systems will probably use an inexpensive > LDO for it. LDO regulators apparently have the feature that they > don't actively drive the voltage down--they wait for other components > in the system to drag the voltage down. Thus they will transition > faster under heavy loads and slower under light loads. What a LDO is doing is basically just charging up a capacitor - the regulation consists of monitoring the voltage on the capacitor and opening a transistor to charge the capacitor when the voltage droops too much. > From experimental evidence, we've seen the voltage change fail if the > card doesn't detect that the voltage fell to less than about 2.3V when > we turn on the clock. On one device (that admittedly had a 47K CMD > pullup instead of a 10K CMD pullup) we saw that the voltage was just > about 2.3V after 5ms and thus the voltage change would sometimes fail. > Doubling the delay gave margin and made the voltage change work 100% > of the time, despite the slightly weaker CMD pull. > At the moment submitting this as an RFC patch since my problem _could_ > be fixed by increasing the pull strength (or using a smaller > capacitor). However being a little bit more lenient to strange > hardware could also be a good thing. Right, and this is probably going beyond the delays that the regulator API is handling since it's not something the regulator hardware is actively managing. --k040vn1t/h12DMPO Content-Type: application/pgp-signature; name="signature.asc" Content-Description: Digital signature -----BEGIN PGP SIGNATURE----- Version: GnuPG v2 iQEcBAEBCAAGBQJVUzDaAAoJECTWi3JdVIfQ02MH/RBbB6L4fYSA4ddDd6GFT05k /fUrtUEgd1uLXg+P8zzgPYVAPwrPB8ySNwZUvIkwy/VI7AFqVkYbKR5u9v3MGc0t xuhLhmOlgVug/HKiyBL3/3RQqeLgR3FI66q7OzoXG7FPOQtvvTsHuUjcrMEmm0sJ /h2PtwraKSW+EN0zyKevEFV5+b/llzImkCPHtaMMdKWDRS7DQ4cq1s72tU1Ojj5g yhQO+bZzW1FnXMotlJZ9x6wD+nA+TLPHIjIOQNFTYIDVhMYH8o6ka6JIP50G/p4H iXdmWgAsqJ5Igb6RDjw6pLc2DxDR2wG02B7Sj7ubmnlu/4F2knLMKbqZBUXXF+E= =PYud -----END PGP SIGNATURE----- --k040vn1t/h12DMPO--