linux-kernel - Re: [PATCH] drm/drm_vblank.c: avoid unsigned int to signed int cast

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Message-ID: <ed008264-e72e-7548-d93e-f7f4130ef2c2@189.cn>
Date:   Tue, 23 May 2023 17:55:39 +0800
From:   Sui Jingfeng <15330273260@....cn>
To:     David Laight <David.Laight@...LAB.COM>,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        Sui Jingfeng <suijingfeng@...ngson.cn>,
        Li Yi <liyi@...ngson.cn>
Cc:     Thomas Zimmermann <tzimmermann@...e.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        "loongson-kernel@...ts.loongnix.cn" 
        <loongson-kernel@...ts.loongnix.cn>
Subject: Re: [PATCH] drm/drm_vblank.c: avoid unsigned int to signed int cast

Hi,

On 2023/5/23 16:50, David Laight wrote:
> From: 15330273260@....cn <15330273260@....cn>
>> Sent: 23 May 2023 05:27
>>
>> On 2023/5/22 19:29, Jani Nikula wrote:
>>> In general, do not use unsigned types in arithmethic to avoid negative
>>> values, because most people will be tripped over by integer promotion
>>> rules, and you'll get negative values anyway.
>>
>> Here I'm not sure about this,
>>
>> but there are plenty unsigned types arithmetic in the kernel.
> The real problem is (attempted) arithmetic on types smaller than int.
> Regardless of whether they are signed or unsigned.

It is about sign extend.

Yes, you may be right. I might create a wrong patch.

But this will not happen in practice, because in general case:

mode->crtc_htotal < 0x8fff;

mode->crtc_vtotal < 0x8fff;

u16 gets promoted to 'signed int' not 'unsigned int'.

Sorry  :/

> 	David
>
> -
> Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
> Registration No: 1397386 (Wales)