From: Raul E Rangel <rran...@chromium.org> This change will allow platform designers better control over signal integrity by allowing them to tune the HS200 and HS400 driver strengths.
The driver strength was previously hard coded to A to solve boot problems with certain platforms. This driver strength does not universally apply to all platforms so we need a knob to adjust it. All older platforms currently have the SDR104 preset hard coded to A in the firmware. This means that switching from the hard coded value in the kernel to reading the SDR104 preset is a no-op for these platforms. Newer platforms will have properly set presets. So this change will support both new and old platforms. Signed-off-by: Raul E Rangel <rran...@chromium.org> Signed-off-by: Victor Ding <victord...@google.com> --- Changes in v2: By Victor Ding <victord...@google.com> - Rebased the patch by using FIELD_GET for preset value bit masks. - (No functional changes). The original patch was developed by Raul E Rangel. https://patchwork.kernel.org/project/linux-mmc/patch/20200928154718.2.Ic6b6031366f090393d00a53fd69e1ada31ceb29e@changeid/ drivers/mmc/host/sdhci-acpi.c | 39 ++++++++++++++++++++++++++++++++--- 1 file changed, 36 insertions(+), 3 deletions(-) diff --git a/drivers/mmc/host/sdhci-acpi.c b/drivers/mmc/host/sdhci-acpi.c index 54205e3be9e8..225cb34cf1b9 100644 --- a/drivers/mmc/host/sdhci-acpi.c +++ b/drivers/mmc/host/sdhci-acpi.c @@ -5,6 +5,7 @@ * Copyright (c) 2012, Intel Corporation. */ +#include <linux/bitfield.h> #include <linux/init.h> #include <linux/export.h> #include <linux/module.h> @@ -545,10 +546,42 @@ struct amd_sdhci_host { static int amd_select_drive_strength(struct mmc_card *card, unsigned int max_dtr, int host_drv, - int card_drv, int *drv_type) + int card_drv, int *host_driver_strength) { - *drv_type = MMC_SET_DRIVER_TYPE_A; - return MMC_SET_DRIVER_TYPE_A; + struct sdhci_host *host = mmc_priv(card->host); + u16 preset, preset_driver_strength; + + /* + * This method is only called by mmc_select_hs200 so we only need to + * read from the HS200 (SDR104) preset register. + * + * Firmware that has "invalid/default" presets return a driver strength + * of A. This matches the previously hard coded value. + */ + preset = sdhci_readw(host, SDHCI_PRESET_FOR_SDR104); + preset_driver_strength = FIELD_GET(SDHCI_PRESET_DRV_MASK, preset); + + /* + * We want the controller driver strength to match the card's driver + * strength so they have similar rise/fall times. + * + * The controller driver strength set by this method is sticky for all + * timings after this method is called. This unfortunately means that + * while HS400 tuning is in progress we end up with mismatched driver + * strengths between the controller and the card. HS400 tuning requires + * switching from HS400->DDR52->HS->HS200->HS400. So the driver mismatch + * happens while in DDR52 and HS modes. This has not been observed to + * cause problems. Enabling presets would fix this issue. + */ + *host_driver_strength = preset_driver_strength; + + /* + * The resulting card driver strength is only set when switching the + * card's timing to HS200 or HS400. The card will use the default driver + * strength (B) for any other mode. + */ + return preset_driver_strength; + } static void sdhci_acpi_amd_hs400_dll(struct sdhci_host *host, bool enable) -- 2.29.0.rc2.309.g374f81d7ae-goog
