linux-kernel - Re: [PATCH 03/34] brcmfmac: firmware: Support having multiple alt paths

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Date:   Mon, 3 Jan 2022 15:17:51 +0900
From:   Hector Martin <marcan@...can.st>
To:     Dmitry Osipenko <digetx@...il.com>,
        Kalle Valo <kvalo@...eaurora.org>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Rob Herring <robh+dt@...nel.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Len Brown <lenb@...nel.org>,
        Arend van Spriel <aspriel@...il.com>,
        Franky Lin <franky.lin@...adcom.com>,
        Hante Meuleman <hante.meuleman@...adcom.com>,
        Chi-hsien Lin <chi-hsien.lin@...ineon.com>,
        Wright Feng <wright.feng@...ineon.com>
Cc:     Sven Peter <sven@...npeter.dev>,
        Alyssa Rosenzweig <alyssa@...enzweig.io>,
        Mark Kettenis <kettenis@...nbsd.org>,
        Rafał Miłecki <zajec5@...il.com>,
        Pieter-Paul Giesberts <pieter-paul.giesberts@...adcom.com>,
        Linus Walleij <linus.walleij@...aro.org>,
        Hans de Goede <hdegoede@...hat.com>,
        "John W. Linville" <linville@...driver.com>,
        "brian m. carlson" <sandals@...stytoothpaste.net>,
        linux-wireless@...r.kernel.org, netdev@...r.kernel.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-acpi@...r.kernel.org, brcm80211-dev-list.pdl@...adcom.com,
        SHA-cyfmac-dev-list@...ineon.com
Subject: Re: [PATCH 03/34] brcmfmac: firmware: Support having multiple alt
 paths

On 2022/01/03 10:26, Dmitry Osipenko wrote:
> 03.01.2022 03:41, Hector Martin пишет:
>>> There is indeed no need for the castings in such cases, it's a typical
>>> code pattern in kernel. You would need to do the casting for the other
>>> way around, i.e. if char ** was returned and **alt_paths was a const.
>> You do need to do the cast. Try it.
>>
>> $ cat test.c
>> int main() {
>>         char *foo[1];
>>         const char **bar = foo;
>>
>>         return 0;
>> }
>>
>> $ gcc test.c
>> test.c: In function ‘main’:
>> test.c:4:28: warning: initialization of ‘const char **’ from
>> incompatible pointer type ‘char **’ [-Wincompatible-pointer-types]
>>     4 |         const char **bar = foo;
>>       |
>>
>> You can implicitly cast char* to const char*, but you *cannot*
>> impliclicitly cast char** to const char** for the reason I explained. It
>> requires a cast.
> 
> Right, I read it as "char * const *". The "const char **" vs "char *
> const *" always confuses me.
> 
> Hence you should've written "const char **alt_paths;" in
> brcm_alt_fw_paths() in the first place and then casting wouldn't have
> been needed.

Sure, in this case that works since the string is just strduped and
never mutated. Either way this will change to an argument instead of a
return value, since I'll change it to be statically sized as you said
and allocated by the caller (or in the struct).

-- 
Hector Martin (marcan@...can.st)
Public Key: https://mrcn.st/pub