linux-kernel - [PATCH v2 04/13] soc: bcm: brcmstb: Document panic notifier action and remove useless header

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Date:   Tue, 19 Jul 2022 16:53:17 -0300
From:   "Guilherme G. Piccoli" <gpiccoli@...lia.com>
To:     akpm@...ux-foundation.org, bhe@...hat.com, pmladek@...e.com,
        kexec@...ts.infradead.org
Cc:     linux-kernel@...r.kernel.org, linux-hyperv@...r.kernel.org,
        netdev@...r.kernel.org, x86@...nel.org, kernel-dev@...lia.com,
        kernel@...ccoli.net, halves@...onical.com, fabiomirmar@...il.com,
        alejandro.j.jimenez@...cle.com, andriy.shevchenko@...ux.intel.com,
        arnd@...db.de, bp@...en8.de, corbet@....net,
        d.hatayama@...fujitsu.com, dave.hansen@...ux.intel.com,
        dyoung@...hat.com, feng.tang@...el.com, gregkh@...uxfoundation.org,
        mikelley@...rosoft.com, hidehiro.kawai.ez@...achi.com,
        jgross@...e.com, john.ogness@...utronix.de, keescook@...omium.org,
        luto@...nel.org, mhiramat@...nel.org, mingo@...hat.com,
        paulmck@...nel.org, peterz@...radead.org, rostedt@...dmis.org,
        senozhatsky@...omium.org, stern@...land.harvard.edu,
        tglx@...utronix.de, vgoyal@...hat.com, vkuznets@...hat.com,
        will@...nel.org, "Guilherme G. Piccoli" <gpiccoli@...lia.com>,
        bcm-kernel-feedback-list@...adcom.com,
        Brian Norris <computersforpeace@...il.com>,
        Doug Berger <opendmb@...il.com>,
        Justin Chen <justinpopo6@...il.com>,
        Lee Jones <lee.jones@...aro.org>,
        Markus Mayer <mmayer@...adcom.com>,
        Florian Fainelli <f.fainelli@...il.com>
Subject: [PATCH v2 04/13] soc: bcm: brcmstb: Document panic notifier action and remove useless header

The panic notifier of this driver is very simple code-wise, just a
memory write to a special position with some numeric code. But this
is not clear from the semantic point-of-view, and there is no public
documentation about that either.

After discussing this in the mailing-lists [0] and having Florian
explained it very well, document that in the code for the future
generations asking the same questions. Also, while at it, remove
a useless header.

[0] https://lore.kernel.org/lkml/781cafb0-8d06-8b56-907a-5175c2da196a@gmail.com

Cc: Brian Norris <computersforpeace@...il.com>
Cc: Doug Berger <opendmb@...il.com>
Cc: Justin Chen <justinpopo6@...il.com>
Cc: Lee Jones <lee.jones@...aro.org>
Cc: Markus Mayer <mmayer@...adcom.com>
Acked-by: Florian Fainelli <f.fainelli@...il.com>
Signed-off-by: Guilherme G. Piccoli <gpiccoli@...lia.com>

---

V2:
- Removed the Fixes tag;
- Added Florian's ACK - thanks!

 drivers/soc/bcm/brcmstb/pm/pm-arm.c | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/drivers/soc/bcm/brcmstb/pm/pm-arm.c b/drivers/soc/bcm/brcmstb/pm/pm-arm.c
index 70ad0f3dce28..2a6adaa29596 100644
--- a/drivers/soc/bcm/brcmstb/pm/pm-arm.c
+++ b/drivers/soc/bcm/brcmstb/pm/pm-arm.c
@@ -25,7 +25,6 @@
 #include <linux/kernel.h>
 #include <linux/memblock.h>
 #include <linux/module.h>
-#include <linux/notifier.h>
 #include <linux/of.h>
 #include <linux/of_address.h>
 #include <linux/panic_notifier.h>
@@ -664,7 +663,20 @@ static void __iomem *brcmstb_ioremap_match(const struct of_device_id *matches,
 
 	return of_io_request_and_map(dn, index, dn->full_name);
 }
-
+/*
+ * The AON is a small domain in the SoC that can retain its state across
+ * various system wide sleep states and specific reset conditions; the
+ * AON DATA RAM is a small RAM of a few words (< 1KB) which can store
+ * persistent information across such events.
+ *
+ * The purpose of the below panic notifier is to help with notifying
+ * the bootloader that a panic occurred and so that it should try its
+ * best to preserve the DRAM contents holding that buffer for recovery
+ * by the kernel as opposed to wiping out DRAM clean again.
+ *
+ * Reference: comment from Florian Fainelli, at
+ * https://lore.kernel.org/lkml/781cafb0-8d06-8b56-907a-5175c2da196a@gmail.com
+ */
 static int brcmstb_pm_panic_notify(struct notifier_block *nb,
 		unsigned long action, void *data)
 {
-- 
2.37.1