mmc: core: Fixup support for writeback-cache for eMMC and SD

During the card initialization process, the mmc core checks whether the
eMMC/SD card supports an internal writeback-cache and then enables it
inside the card.

Unfortunately, this isn't according to what the mmc core reports to the
upper block layer. Instead, the writeback-cache support with REQ_FLUSH and
REQ_FUA, are being enabled depending on whether the host supports the CMD23
(MMC_CAP_CMD23) and whether an eMMC supports the reliable-write command.

This is wrong and it may also sound awkward. In fact, it's a remnant
from when both eMMC/SD cards didn't have dedicated commands/support to
control the internal writeback-cache. In other words, it was the best we
could do at that point in time.

To fix the problem, but also without breaking backwards compatibility,
let's align the REQ_FLUSH support with whether the writeback-cache became
successfully enabled - for both eMMC and SD cards.

Cc: stable@kernel.org
Fixes: 881d1c2 ("mmc: core: Add cache control for eMMC4.5 device")
Fixes: 130206a ("mmc: core: Add support for cache ctrl for SD cards")
Depends-on: 97fce12 ("mmc: block: Issue a cache flush only when it's enabled")
Reviewed-by: Avri Altman <Avri.Altman@wdc.com>
Signed-off-by: Michael Wu <michael@allwinnertech.com>
Link: https://lore.kernel.org/r/20220331073223.106415-1-michael@allwinnertech.com
[Ulf: Re-wrote the commit message]
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>

Author: Michael Wu

drivers/mmc/core/block.c

@@ -2382,6 +2382,8 @@ static struct mmc_blk_data *mmc_blk_alloc_req(struct mmc_card *card,
 	struct mmc_blk_data *md;
 	int devidx, ret;
 	char cap_str[10];
+	bool cache_enabled = false;
+	bool fua_enabled = false;
 
 	devidx = ida_simple_get(&mmc_blk_ida, 0, max_devices, GFP_KERNEL);
 	if (devidx < 0) {
@@ -2461,13 +2463,17 @@ static struct mmc_blk_data *mmc_blk_alloc_req(struct mmc_card *card,
 			md->flags |= MMC_BLK_CMD23;
 	}
 
-	if (mmc_card_mmc(card) &&
-	    md->flags & MMC_BLK_CMD23 &&
+	if (md->flags & MMC_BLK_CMD23 &&
 	    ((card->ext_csd.rel_param & EXT_CSD_WR_REL_PARAM_EN) ||
 	     card->ext_csd.rel_sectors)) {
 		md->flags |= MMC_BLK_REL_WR;
-		blk_queue_write_cache(md->queue.queue, true, true);
+		fua_enabled = true;
+		cache_enabled = true;
 	}
+	if (mmc_cache_enabled(card->host))
+		cache_enabled  = true;
+
+	blk_queue_write_cache(md->queue.queue, cache_enabled, fua_enabled);
 
 	string_get_size((u64)size, 512, STRING_UNITS_2,
 			cap_str, sizeof(cap_str));