mmc: core: Add MMC Command Queue Support kernel parameter

This parameter offers a workaround for cards that report command queue
(CMDQ) support but don't work correctly when CMDQ is enabled. At least
some ROCKPro64 + Foresee (32GB) eMMC card combinations have trouble
working correctly. Setting mmc_cmdqueue=off in kernel command line
disables CMDQ support and may help with troublesome hardware.

Author: Jari Hämäläinen

Documentation/admin-guide/kernel-parameters.txt

@@ -2506,6 +2506,14 @@
 			log everything. Information is printed at KERN_DEBUG
 			so loglevel=8 may also need to be specified.
 
+	mmc_cmdqueue= [MMC]
+			Enable or disable MMC command queue (CMDQ) support. When
+			enabled MMC driver will try to enable CMDQ for cards that
+			support it. When disabled CMDQ will not be enabled for any
+			card.
+			Format: <bool>  (1/y/on=enable, 0/n/off=disable)
+			default: enabled
+
 	module.sig_enforce
 			[KNL] When CONFIG_MODULE_SIG is set, this means that
 			modules without (valid) signatures will fail to load.

drivers/mmc/core/mmc.c

@@ -9,7 +9,7 @@
  * it under the terms of the GNU General Public License version 2 as
  * published by the Free Software Foundation.
  */
-
+#include <linux/moduleparam.h>
 #include <linux/err.h>
 #include <linux/of.h>
 #include <linux/slab.h>
@@ -65,6 +65,10 @@ static const unsigned int taac_mant[] = {
 		__res & __mask;						\
 	})
 
+/* Enable / disable command queue support */
+static bool mmc_cmdqueue_support = true;
+core_param(mmc_cmdqueue, mmc_cmdqueue_support, bool, S_IRUGO);
+
 /*
  * Given the decoded CSD structure, decode the raw CID to our CID structure.
  */
@@ -1824,15 +1828,20 @@ static int mmc_init_card(struct mmc_host *host, u32 ocr,
 	 */
 	card->ext_csd.cmdq_en = false;
 	if (card->ext_csd.cmdq_support && host->caps2 & MMC_CAP2_CQE) {
-		err = mmc_cmdq_enable(card);
-		if (err && err != -EBADMSG)
-			goto free_card;
-		if (err) {
-			pr_warn("%s: Enabling CMDQ failed\n",
-				mmc_hostname(card->host));
-			card->ext_csd.cmdq_support = false;
-			card->ext_csd.cmdq_depth = 0;
-			err = 0;
+		if (mmc_cmdqueue_support) {
+			err = mmc_cmdq_enable(card);
+			if (err && err != -EBADMSG)
+				 goto free_card;
+			if (err) {
+				pr_warn("%s: Enabling CMDQ failed\n",
+					mmc_hostname(card->host));
+				card->ext_csd.cmdq_support = false;
+				card->ext_csd.cmdq_depth = 0;
+				err = 0;
+			}
+		} else {
+			pr_info("%s: CMDQ support disabled in kernel\n",
+				mmc_hostname(host));
 		}
 	}
 	/*