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Subject[PATCH v2] mmc: sdhci-acpi: AMDI0040: Allow changing HS200/HS400 driver strength
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From: Raul E Rangel <rrangel@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 <rrangel@chromium.org>
Signed-off-by: Victor Ding <victording@google.com>

---

Changes in v2:
By Victor Ding <victording@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
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