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>
---
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 d335a34ad05b3..5c9a041af5b4b 100644
--- a/drivers/mmc/host/sdhci-acpi.c
+++ b/drivers/mmc/host/sdhci-acpi.c
@@ -545,10 +545,43 @@ 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 =
+ (preset & SDHCI_PRESET_DRV_MASK) >> SDHCI_PRESET_DRV_SHIFT;
+
+ /*
+ * 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)
--
2.28.0.709.gb0816b6eb0-goog
