FAILED: patch "[PATCH] firmware: qcom: scm: Fix missing read barrier in" failed to apply to 6.6-stable tree

The patch below does not apply to the 6.6-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.6.y
git checkout FETCH_HEAD
git cherry-pick -x 0a744cceebd0480cb39587b3b1339d66a9d14063
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2025021032-maturing-punctuate-10e5@gregkh' --subject-prefix 'PATCH 6.6.y' HEAD^..

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 0a744cceebd0480cb39587b3b1339d66a9d14063 Mon Sep 17 00:00:00 2001
From: Krzysztof Kozlowski <[email protected]>
Date: Mon, 9 Dec 2024 15:27:54 +0100
Subject: [PATCH] firmware: qcom: scm: Fix missing read barrier in
qcom_scm_is_available()

Commit 2e4955167ec5 ("firmware: qcom: scm: Fix __scm and waitq completion variable initialization") introduced a write barrier in probe function to store global '__scm' variable. It also claimed that it added a read barrier, because as we all known barriers are paired (see memory-barriers.txt: "Note that write barriers should normally be paired with read or address-dependency barriers"), however it did not really add it.

The offending commit used READ_ONCE() to access '__scm' global which is not a barrier.

The barrier is needed so the store to '__scm' will be properly visible. This is most likely not fatal in current driver design, because missing read barrier would mean qcom_scm_is_available() callers will access old value, NULL. Driver does not support unbinding and does not correctly handle probe failures, thus there is no risk of stale or old pointer in '__scm' variable.

However for code correctness, readability and to be sure that we did not mess up something in this tricky topic of SMP barriers, add a read barrier for accessing '__scm'. Change also comment from useless/obvious what does barrier do, to what is expected: which other parts of the code are involved here.

index e736b4b46ea4..cde60566c793 100644
@@ -1872,7 +1872,8 @@ static int qcom_scm_qseecom_init(struct qcom_scm *scm)
  */
 bool qcom_scm_is_available(void)
 {
-	return !!READ_ONCE(__scm);
+	/* Paired with smp_store_release() in qcom_scm_probe */
+	return !!smp_load_acquire(&__scm);
 }
 EXPORT_SYMBOL_GPL(qcom_scm_is_available);
 

@@ -2029,7 +2030,7 @@ static int qcom_scm_probe(struct platform_device *pdev)
 	if (ret)
 		return ret;
 
-	/* Let all above stores be available after this */
+	/* Paired with smp_load_acquire() in qcom_scm_is_available(). */
 	smp_store_release(&__scm, scm);
 
 	irq = platform_get_irq_optional(pdev, 0);