From mboxrd@z Thu Jan 1 00:00:00 1970 From: mperttunen@nvidia.com (Mikko Perttunen) Date: Tue, 29 Jul 2014 11:30:08 +0300 Subject: [PATCH 5/8] of: Add Tegra124 EMC bindings In-Reply-To: <53CEA093.6060106@wwwdotorg.org> References: <1405088313-20048-1-git-send-email-mperttunen@nvidia.com> <1405088313-20048-6-git-send-email-mperttunen@nvidia.com> <53CD860B.7030800@wwwdotorg.org> <53CE9514.1050903@wwwdotorg.org> <53CEA093.6060106@wwwdotorg.org> Message-ID: <53D75B90.7050501@nvidia.com> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org Looks like the TRM doesn't document this either. I'll add an option ("nvidia,short-ram-code" ?) for the next version. On 22/07/14 20:34, Stephen Warren wrote: > On 07/22/2014 11:22 AM, Andrew Bresticker wrote: >> On Tue, Jul 22, 2014 at 9:45 AM, Stephen Warren wrote: >>> >>> Does the bootloader adjust the DT that's passed to the kernel so that >>> only the relevant single set of EMC timings is contained in the DT? >> >> No, the DT contains all possible EMC timings for that board. >> >>> On a system where the boot ROM initializes RAM, and where the HW design >>> might have multiple SDRAM configuration, here's what usually happens: >>> >>> a) The BCT contains N sets of SDRAM configurations. >>> >>> b) The boot ROM reads the SDRAM strapping bits, and uses them to pick >>> the correct SDRAM configuration from the N sets in the BCT. >>> >>> c) The kernel DT has N sets of SDRAM configurations. >>> >>> d) The kernel reads the SDRAM strapping bits, and uses them to pick the >>> correct SDRAM configuration from the N sets in the DT. >>> >>> On the ChromeOS boards (so (a) and (b) above are irrelevant) where N is >>> too large to fit into APBDEV_PMC_STRAPPING_OPT_A_0[7:4], (c) and (d) >>> won't work. I assume the kernel DT therefore must be adjusted to only >>> contain the single SDRAM configuration that is relevant for the current HW? >>> >>> (isn't STRAPPING_OPT_A split into 2 2-bit fields; 2 bits for SDRAM index >>> and 2 bits for boot flash index, so max N is quite small?) >> >> Right, there are normally only 2 SDRAM strapping bits available. >> ChromeOS gets around this by having 4 identical boot device entries in >> the BCT, so all possible values of STRAPPING_OPT_A[7:6] map to the >> same boot device. This allows us to use all 4 strapping bits in >> coreboot to pick the SDRAM configuration. > > OK, that explains how it works. > > But that means that when the kernel reads the strapping options, it will > have to know if it uses just 2 bits (standard) or all 4 bits > (non-standard) to index into the DT's array of SDRAM configurations. > We'll need a DT property to represent that. > -- > To unsubscribe from this list: send the line "unsubscribe linux-tegra" in > the body of a message to majordomo at vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html >