Hi Eugeniu, thanks for your patch! On Tue, Nov 5, 2019 at 6:50 AM Eugeniu Rosca <erosca@xxxxxxxxxxxxxx> wrote: > A certain eMMC manufacturer provided below requirement: > ---snip--- > Use "drive strength" value of 4 or 1 for HS400 or 0 for HS200. > ---snip--- > > The existing "fixed-emmc-driver-type" property [1] is the closest one > to implement the above, but it falls short due to being unable to define > two values to differentiate between HS200 and HS400 (both modes may be > supported by the same non-removable MMC device). > > To allow users to set a preferred HS200/HS400 "drive strength", provide > two more bindings inspired from [1]: > - fixed-emmc-driver-type-hs200 > - fixed-emmc-driver-type-hs400 I am sorry that I do not quite understand but as pin control maintainer I am of course triggered by the talk about selecting "drive strength". In my book this means that the pad driver on the chip, driving the line low/high with push-pull (totempole output, usually) is connecting more driver stages, usually just shunting in more totempoles. (Ref https://en.wikipedia.org/wiki/Push%E2%80%93pull_output) If say one totempole gives 2mA drive strength then 4 totempoles gives 8mA drive strength. Are we on the same page here that this is what physically happens? Usually selection of drive strength is done with the pin control framework, so this would need to be backed by code (not in this patch set) that select pin control states that reconfigure the SoC pad drivers to use the requested strength. Alternatively, the (e)MMC block would implement this control directly, but I doubt it. Please clarify which hardware is eventually going to provide the drive strength alteration, because I just don't see it in the patch set. Is the assumption that the (e)MMC hardware will do this autonomously or something? That may be a pecularity to the hardware you're using in that case. I find the fixed-emmc-driver-type-* assignment a but puzzling to be honest, isnt' the driver device tree already specifying what the hardware can do with all of these: mmc-ddr-1_2v mmc-ddr-1_8v mmc-ddr-3_3v mmc-hs200-1_2v mmc-hs200-1_8v mmc-hs400-1_2v mmc-hs400-1_8v mmc-hs400-enhanced-strobe If the host is already specifying mmc-hs200-* or mmc-hs400-* then certainly it should be implied that the host supports hs200 and hs400 and there is no need for the fixed-emmc-driver-type-hs* properties. The code detects when to use each mode and that is when you can insert the code to switch drive strengths, whether using the pin control framework or something else. So to me it seems these DT properties are just introduced to hammer down a certain usecase instead of letting the code with the help of DT speed capabilities flags determine what speed is to be used and select the appropriate drive strength. Yours, Linus Walleij
