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From: Hector Martin <marcan@marcan.st>
To: Dmitry Osipenko <digetx@gmail.com>,
	Kalle Valo <kvalo@codeaurora.org>,
	"David S. Miller" <davem@davemloft.net>,
	Jakub Kicinski <kuba@kernel.org>,
	Rob Herring <robh+dt@kernel.org>,
	"Rafael J. Wysocki" <rafael@kernel.org>,
	Len Brown <lenb@kernel.org>, Arend van Spriel <aspriel@gmail.com>,
	Franky Lin <franky.lin@broadcom.com>,
	Hante Meuleman <hante.meuleman@broadcom.com>,
	Chi-hsien Lin <chi-hsien.lin@infineon.com>,
	Wright Feng <wright.feng@infineon.com>
Cc: "Sven Peter" <sven@svenpeter.dev>,
	"Alyssa Rosenzweig" <alyssa@rosenzweig.io>,
	"Mark Kettenis" <kettenis@openbsd.org>,
	"Rafał Miłecki" <zajec5@gmail.com>,
	"Pieter-Paul Giesberts" <pieter-paul.giesberts@broadcom.com>,
	"Linus Walleij" <linus.walleij@linaro.org>,
	"Hans de Goede" <hdegoede@redhat.com>,
	"John W. Linville" <linville@tuxdriver.com>,
	"brian m. carlson" <sandals@crustytoothpaste.net>,
	linux-wireless@vger.kernel.org, netdev@vger.kernel.org,
	devicetree@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-acpi@vger.kernel.org, brcm80211-dev-list.pdl@broadcom.com,
	SHA-cyfmac-dev-list@infineon.com
Subject: Re: [PATCH 03/34] brcmfmac: firmware: Support having multiple alt paths
Date: Mon, 3 Jan 2022 09:41:27 +0900	[thread overview]
Message-ID: <d96fe60e-c029-b400-9c29-0f95c3632301@marcan.st> (raw)
In-Reply-To: <48f16559-6891-9401-dd8e-762c7573304c@gmail.com>

On 03/01/2022 05.11, Dmitry Osipenko wrote:
> 02.01.2022 17:18, Hector Martin пишет:
>> On 2022/01/02 15:45, Dmitry Osipenko wrote:
>>> 26.12.2021 18:35, Hector Martin пишет:
>>>> -static char *brcm_alt_fw_path(const char *path, const char *board_type)
>>>> +static const char **brcm_alt_fw_paths(const char *path, const char *board_type)
>>>>  {
>>>>  	char alt_path[BRCMF_FW_NAME_LEN];
>>>> +	char **alt_paths;
>>>>  	char suffix[5];
>>>>  
>>>>  	strscpy(alt_path, path, BRCMF_FW_NAME_LEN);
>>>> @@ -609,27 +612,46 @@ static char *brcm_alt_fw_path(const char *path, const char *board_type)
>>>>  	strlcat(alt_path, board_type, BRCMF_FW_NAME_LEN);
>>>>  	strlcat(alt_path, suffix, BRCMF_FW_NAME_LEN);
>>>>  
>>>> -	return kstrdup(alt_path, GFP_KERNEL);
>>>> +	alt_paths = kzalloc(sizeof(char *) * 2, GFP_KERNEL);
>>>
>>> array_size()?
>>
>> Of what array?
> 
> array_size(sizeof(*alt_paths), 2)

Heh, TIL. I thought you meant ARRAY_SIZE. First time I see the lowercase
macro. That's a confusing name collision...

>>>> +	alt_paths[0] = kstrdup(alt_path, GFP_KERNEL);
>>>> +
>>>> +	return (const char **)alt_paths;
>>>
>>> Why this casting is needed?
>>
>> Because implicit conversion from char ** to const char ** is not legal
>> in C, as that could cause const unsoundness if you do this:
>>
>> char *foo[1];
>> const char **bar = foo;
>>
>> bar[0] = "constant string";
>> foo[0][0] = '!'; // clobbers constant string
> 
> It's up to a programmer to decide what is right to do. C gives you
> flexibility, meanwhile it's easy to shoot yourself in the foot if you
> won't be careful.

Which is why that conversion is illegal without a cast and you need to
explicitly choose to shoot yourself in the foot :-)

>> But it's fine in this case since the non-const pointer disappears so
>> nothing can ever write through it again.
>>
> 
> 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.

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

  reply	other threads:[~2022-01-03  0:41 UTC|newest]

Thread overview: 105+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-12-26 15:35 [RFC PATCH 00/34] brcmfmac: Support Apple T2 and M1 platforms Hector Martin
2021-12-26 15:35 ` [PATCH 01/34] dt-bindings: net: bcm4329-fmac: Add Apple properties & chips Hector Martin
2021-12-26 21:02   ` Linus Walleij
2021-12-26 23:34   ` Rob Herring
2021-12-27 16:36   ` Rob Herring
2021-12-27 17:23     ` Hector Martin
2021-12-29 16:38       ` Mark Kettenis
2022-01-02 14:12         ` Hector Martin
2021-12-29 16:42   ` Mark Kettenis
2022-01-04  5:47     ` Hector Martin
2021-12-26 15:35 ` [PATCH 02/34] brcmfmac: pcie: Declare missing firmware files in pcie.c Hector Martin
2021-12-26 21:04   ` Linus Walleij
2021-12-26 15:35 ` [PATCH 03/34] brcmfmac: firmware: Support having multiple alt paths Hector Martin
2022-01-02  5:31   ` Linus Walleij
2022-01-02  7:10     ` Dmitry Osipenko
2022-01-02  6:38   ` Dmitry Osipenko
2022-01-02  6:45   ` Dmitry Osipenko
2022-01-02 14:18     ` Hector Martin
2022-01-02 20:11       ` Dmitry Osipenko
2022-01-03  0:41         ` Hector Martin [this message]
2022-01-03  1:26           ` Dmitry Osipenko
2022-01-03  6:17             ` Hector Martin
2022-01-02  6:55   ` Dmitry Osipenko
2022-01-03  6:18     ` Hector Martin
2022-01-02  7:08   ` Dmitry Osipenko
2022-01-02  7:20     ` Dmitry Osipenko
2022-01-02 14:25     ` Hector Martin
2022-01-02 20:12       ` Dmitry Osipenko
2021-12-26 15:35 ` [PATCH 04/34] brcmfmac: firmware: Handle per-board clm_blob files Hector Martin
2022-01-02  6:21   ` Linus Walleij
2021-12-26 15:35 ` [PATCH 05/34] brcmfmac: pcie/sdio/usb: Get CLM blob via standard firmware mechanism Hector Martin
2022-01-02  6:22   ` Linus Walleij
2021-12-26 15:35 ` [PATCH 06/34] brcmfmac: firmware: Support passing in multiple board_types Hector Martin
2022-01-02  5:34   ` Linus Walleij
2021-12-26 15:35 ` [PATCH 07/34] brcmfmac: pcie: Read Apple OTP information Hector Martin
2022-01-02  5:38   ` Linus Walleij
2022-01-03  5:51     ` Hector Martin
2022-01-03 11:13       ` Linus Walleij
2021-12-26 15:35 ` [PATCH 08/34] brcmfmac: of: Fetch Apple properties Hector Martin
2022-01-02  5:40   ` Linus Walleij
2021-12-26 15:35 ` [PATCH 09/34] brcmfmac: pcie: Perform firmware selection for Apple platforms Hector Martin
2022-01-02  5:44   ` Linus Walleij
2021-12-26 15:36 ` [PATCH 10/34] brcmfmac: firmware: Allow platform to override macaddr Hector Martin
2022-01-02  5:50   ` Linus Walleij
2022-01-03  5:42     ` Hector Martin
2021-12-26 15:36 ` [PATCH 11/34] brcmfmac: msgbuf: Increase RX ring sizes to 1024 Hector Martin
2022-01-02  5:50   ` Linus Walleij
2021-12-26 15:36 ` [PATCH 12/34] brcmfmac: pcie: Fix crashes due to early IRQs Hector Martin
2022-01-02  5:51   ` Linus Walleij
2021-12-26 15:36 ` [PATCH 13/34] brcmfmac: pcie: Support PCIe core revisions >= 64 Hector Martin
2022-01-02  5:53   ` Linus Walleij
2021-12-26 15:36 ` [PATCH 14/34] brcmfmac: pcie: Add IDs/properties for BCM4378 Hector Martin
2022-01-02  5:53   ` Linus Walleij
2021-12-26 15:36 ` [PATCH 15/34] ACPI / property: Support strings in Apple _DSM props Hector Martin
2021-12-26 18:20   ` Lukas Wunner
2022-01-02  6:20   ` Linus Walleij
2021-12-26 15:36 ` [PATCH 16/34] brcmfmac: acpi: Add support for fetching Apple ACPI properties Hector Martin
2022-01-02  5:58   ` Linus Walleij
2022-01-03  6:03     ` Hector Martin
2022-01-03 11:14       ` Linus Walleij
     [not found]   ` <CAHp75VcZcJ+zCDL-J+w8gEeKXGYdJajjLoa1JTj_kkJixrV12Q@mail.gmail.com>
2022-01-03 17:22     ` Hector Martin
     [not found]       ` <CAHp75Vedgs_zTH2O120jtUuQiuseA0VN62TJiJ7kAi1f5nDQ6Q@mail.gmail.com>
2022-01-04  5:22         ` Hector Martin
2022-01-10  9:59     ` Kalle Valo
2021-12-26 15:36 ` [PATCH 17/34] brcmfmac: pcie: Provide a buffer of random bytes to the device Hector Martin
2022-01-02  5:59   ` Linus Walleij
2021-12-26 15:36 ` [PATCH 18/34] brcmfmac: pcie: Add IDs/properties for BCM4355 Hector Martin
2022-01-02  6:00   ` Linus Walleij
2021-12-26 15:36 ` [PATCH 19/34] brcmfmac: pcie: Add IDs/properties for BCM4377 Hector Martin
2022-01-02  6:01   ` Linus Walleij
2021-12-26 15:36 ` [PATCH 20/34] brcmfmac: pcie: Perform correct BCM4364 firmware selection Hector Martin
2022-01-02  6:02   ` Linus Walleij
2021-12-26 15:36 ` [PATCH 21/34] brcmfmac: chip: Only disable D11 cores; handle an arbitrary number Hector Martin
2022-01-02  6:03   ` Linus Walleij
2021-12-26 15:36 ` [PATCH 22/34] brcmfmac: chip: Handle 1024-unit sizes for TCM blocks Hector Martin
2022-01-02  6:09   ` Linus Walleij
2021-12-26 15:36 ` [PATCH 23/34] brcmfmac: cfg80211: Add support for scan params v2 Hector Martin
2022-01-02  6:23   ` Linus Walleij
2021-12-26 15:36 ` [PATCH 24/34] brcmfmac: feature: Add support for setting feats based on WLC version Hector Martin
2022-01-02  6:11   ` Linus Walleij
2021-12-26 15:36 ` [PATCH 25/34] brcmfmac: cfg80211: Add support for PMKID_V3 operations Hector Martin
2022-01-02  6:12   ` Linus Walleij
2021-12-26 15:36 ` [PATCH 26/34] brcmfmac: cfg80211: Pass the PMK in binary instead of hex Hector Martin
2022-01-02  6:13   ` Linus Walleij
2021-12-26 15:36 ` [PATCH 27/34] brcmfmac: pcie: Add IDs/properties for BCM4387 Hector Martin
2022-01-02  6:13   ` Linus Walleij
2021-12-26 15:36 ` [PATCH 28/34] brcmfmac: pcie: Replace brcmf_pcie_copy_mem_todev with memcpy_toio Hector Martin
2022-01-02  6:15   ` Linus Walleij
2021-12-26 15:36 ` [PATCH 29/34] brcmfmac: pcie: Read the console on init and shutdown Hector Martin
2022-01-02  6:16   ` Linus Walleij
2021-12-26 15:36 ` [PATCH 30/34] brcmfmac: pcie: Release firmwares in the brcmf_pcie_setup error path Hector Martin
2022-01-02  6:16   ` Linus Walleij
2021-12-26 15:36 ` [PATCH 31/34] brcmfmac: fwil: Constify iovar name arguments Hector Martin
2022-01-02  6:17   ` Linus Walleij
2021-12-26 15:36 ` [PATCH 32/34] brcmfmac: common: Add support for downloading TxCap blobs Hector Martin
2022-01-02  6:18   ` Linus Walleij
2021-12-26 15:36 ` [PATCH 33/34] brcmfmac: pcie: Load and provide " Hector Martin
2022-01-02  6:19   ` Linus Walleij
2021-12-26 15:36 ` [PATCH 34/34] brcmfmac: common: Add support for external calibration blobs Hector Martin
2022-01-02  6:19   ` Linus Walleij
2021-12-26 19:17 ` [RFC PATCH 00/34] brcmfmac: Support Apple T2 and M1 platforms Lukas Wunner
2021-12-26 21:42   ` Hans de Goede
2021-12-27 11:53     ` Hector Martin
2022-01-02  6:25 ` Linus Walleij
2022-01-03  6:27   ` Hector Martin
2022-01-03 10:20     ` Arend van Spriel

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