linux-kernel - Re:Re: [PATCH] memory/tegra: Add missing of_node_get() in tegra_emc_find_node_by_ram

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Date:   Wed, 22 Jun 2022 14:56:54 +0800 (CST)
From:   "Liang He" <windhl@....com>
To:     "Krzysztof Kozlowski" <krzysztof.kozlowski@...aro.org>
Cc:     thierry.reding@...il.com, jonathanh@...dia.com,
        linux-kernel@...r.kernel.org, linux-tegra@...r.kernel.org
Subject: Re:Re: [PATCH] memory/tegra: Add missing of_node_get() in
 tegra_emc_find_node_by_ram_code



At 2022-06-22 14:50:41, "Krzysztof Kozlowski" <krzysztof.kozlowski@...aro.org> wrote:
>On 22/06/2022 06:28, Liang He wrote:
>> of_find_node_by_name() will decrease the refcount of its first arg and
>> we need to add a of_node_put() to keep refcount balance.
>
>We do NOT need to add an of_node_put?
>

Sorry, this is a typo, I want to say we need to add a 'of_node_get()'

>Anyway why do you add of_node_get which is not explained here?
>

This is a common mistake that missing the of_node_get(X) before we call
of_find_node_by_name(X,..) as the of_find_xxx will decrease the refcount of X.

A similar error is:

https://lore.kernel.org/all/20200720152806.443262648@linuxfoundation.org/

If I am wrong, please correct me, thanks.
>> 
>> Besides, during the 'for' loop execution, the refcount of 'np' will be
>> automatically increased and decreased. There is no need to call
>> of_node_put() again.
>> 
>> Signed-off-by: Liang He <windhl@....com>
>> ---
>>  drivers/memory/tegra/tegra20-emc.c | 3 ++-
>>  1 file changed, 2 insertions(+), 1 deletion(-)
>> 
>> diff --git a/drivers/memory/tegra/tegra20-emc.c b/drivers/memory/tegra/tegra20-emc.c
>> index 25ba3c5e4ad6..e43a5f8e188e 100644
>> --- a/drivers/memory/tegra/tegra20-emc.c
>> +++ b/drivers/memory/tegra/tegra20-emc.c
>> @@ -477,6 +477,7 @@ tegra_emc_find_node_by_ram_code(struct tegra_emc *emc)
>>  
>>  	ram_code = tegra_read_ram_code();
>>  
>> +	of_get_node(dev->of_node);
>>  	for (np = of_find_node_by_name(dev->of_node, "emc-tables"); np;
>>  	     np = of_find_node_by_name(np, "emc-tables")) {
>>  		err = of_property_read_u32(np, "nvidia,ram-code", &value);
>> @@ -484,6 +485,7 @@ tegra_emc_find_node_by_ram_code(struct tegra_emc *emc)
>>  			struct device_node *lpddr2_np;
>>  			bool cfg_mismatches = false;
>>  
>> +			of_node_get(np);
>>  			lpddr2_np = of_find_node_by_name(np, "lpddr2");
>>  			if (lpddr2_np) {
>>  				const struct lpddr2_info *info;
>> @@ -521,7 +523,6 @@ tegra_emc_find_node_by_ram_code(struct tegra_emc *emc)
>>  			}
>>  
>>  			if (cfg_mismatches) {
>> -				of_node_put(np);
>>  				continue;
>>  			}
>>  		}
>
>
>Best regards,
>Krzysztof