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([2600:1700:e321:62f0:da43:aeff:fecc:bfd5]) by smtp.gmail.com with ESMTPSA id d9443c01a7336-21c2ceb9e7dsm61142095ad.77.2025.01.20.06.15.32 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 20 Jan 2025 06:15:33 -0800 (PST) Sender: Guenter Roeck Message-ID: <0a65de6c-74d5-4d3e-be75-0aa9ecc82da1@roeck-us.net> Date: Mon, 20 Jan 2025 06:15:30 -0800 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: Buiild error in i915/xe To: Jani Nikula , David Laight Cc: Linus Torvalds , David Laight , Arnd Bergmann , "linux-kernel@vger.kernel.org" , Jens Axboe , Matthew Wilcox , Christoph Hellwig , Andrew Morton , Andy Shevchenko , Dan Carpenter , "Jason A . Donenfeld" , "pedro.falcato@gmail.com" , Mateusz Guzik , "linux-mm@kvack.org" , Lorenzo Stoakes , intel-xe@lists.freedesktop.org, intel-gfx@lists.freedesktop.org, David Airlie , Simona Vetter , Rodrigo Vivi References: <34d53778977747f19cce2abb287bb3e6@AcuMS.aculab.com> <20250118170959.3aa56f4d@pumpkin> <29ef57a1-e4dd-4d5d-8726-f1f79c698b66@roeck-us.net> <20250118221123.5bb65e64@pumpkin> <20250119090935.7c690f85@pumpkin> <87ed0xrcb8.fsf@intel.com> <20250120111551.435176c4@pumpkin> <878qr5ras2.fsf@intel.com> Content-Language: en-US From: Guenter Roeck Autocrypt: addr=linux@roeck-us.net; keydata= xsFNBE6H1WcBEACu6jIcw5kZ5dGeJ7E7B2uweQR/4FGxH10/H1O1+ApmcQ9i87XdZQiB9cpN RYHA7RCEK2dh6dDccykQk3bC90xXMPg+O3R+C/SkwcnUak1UZaeK/SwQbq/t0tkMzYDRxfJ7 nyFiKxUehbNF3r9qlJgPqONwX5vJy4/GvDHdddSCxV41P/ejsZ8PykxyJs98UWhF54tGRWFl 7i1xvaDB9lN5WTLRKSO7wICuLiSz5WZHXMkyF4d+/O5ll7yz/o/JxK5vO/sduYDIlFTvBZDh gzaEtNf5tQjsjG4io8E0Yq0ViobLkS2RTNZT8ICq/Jmvl0SpbHRvYwa2DhNsK0YjHFQBB0FX IdhdUEzNefcNcYvqigJpdICoP2e4yJSyflHFO4dr0OrdnGLe1Zi/8Xo/2+M1dSSEt196rXaC kwu2KgIgmkRBb3cp2vIBBIIowU8W3qC1+w+RdMUrZxKGWJ3juwcgveJlzMpMZNyM1jobSXZ0 VHGMNJ3MwXlrEFPXaYJgibcg6brM6wGfX/LBvc/haWw4yO24lT5eitm4UBdIy9pKkKmHHh7s jfZJkB5fWKVdoCv/omy6UyH6ykLOPFugl+hVL2Prf8xrXuZe1CMS7ID9Lc8FaL1ROIN/W8Vk BIsJMaWOhks//7d92Uf3EArDlDShwR2+D+AMon8NULuLBHiEUQARAQABzTJHdWVudGVyIFJv ZWNrIChMaW51eCBhY2NvdW50KSA8bGludXhAcm9lY2stdXMubmV0PsLBgQQTAQIAKwIbAwYL CQgHAwIGFQgCCQoLBBYCAwECHgECF4ACGQEFAlVcphcFCRmg06EACgkQyx8mb86fmYFg0RAA nzXJzuPkLJaOmSIzPAqqnutACchT/meCOgMEpS5oLf6xn5ySZkl23OxuhpMZTVX+49c9pvBx hpvl5bCWFu5qC1jC2eWRYU+aZZE4sxMaAGeWenQJsiG9lP8wkfCJP3ockNu0ZXXAXwIbY1O1 c+l11zQkZw89zNgWgKobKzrDMBFOYtAh0pAInZ9TSn7oA4Ctejouo5wUugmk8MrDtUVXmEA9 7f9fgKYSwl/H7dfKKsS1bDOpyJlqhEAH94BHJdK/b1tzwJCFAXFhMlmlbYEk8kWjcxQgDWMu GAthQzSuAyhqyZwFcOlMCNbAcTSQawSo3B9yM9mHJne5RrAbVz4TWLnEaX8gA5xK3uCNCeyI sqYuzA4OzcMwnnTASvzsGZoYHTFP3DQwf2nzxD6yBGCfwNGIYfS0i8YN8XcBgEcDFMWpOQhT Pu3HeztMnF3HXrc0t7e5rDW9zCh3k2PA6D2NV4fews9KDFhLlTfCVzf0PS1dRVVWM+4jVl6l HRIAgWp+2/f8dx5vPc4Ycp4IsZN0l1h9uT7qm1KTwz+sSl1zOqKD/BpfGNZfLRRxrXthvvY8 BltcuZ4+PGFTcRkMytUbMDFMF9Cjd2W9dXD35PEtvj8wnEyzIos8bbgtLrGTv/SYhmPpahJA l8hPhYvmAvpOmusUUyB30StsHIU2LLccUPPOwU0ETofVZwEQALlLbQeBDTDbwQYrj0gbx3bq 7kpKABxN2MqeuqGr02DpS9883d/t7ontxasXoEz2GTioevvRmllJlPQERVxM8gQoNg22twF7 pB/zsrIjxkE9heE4wYfN1AyzT+AxgYN6f8hVQ7Nrc9XgZZe+8IkuW/Nf64KzNJXnSH4u6nJM J2+Dt274YoFcXR1nG76Q259mKwzbCukKbd6piL+VsT/qBrLhZe9Ivbjq5WMdkQKnP7gYKCAi pNVJC4enWfivZsYupMd9qn7Uv/oCZDYoBTdMSBUblaLMwlcjnPpOYK5rfHvC4opxl+P/Vzyz 6WC2TLkPtKvYvXmdsI6rnEI4Uucg0Au/Ulg7aqqKhzGPIbVaL+U0Wk82nz6hz+WP2ggTrY1w ZlPlRt8WM9w6WfLf2j+PuGklj37m+KvaOEfLsF1v464dSpy1tQVHhhp8LFTxh/6RWkRIR2uF I4v3Xu/k5D0LhaZHpQ4C+xKsQxpTGuYh2tnRaRL14YMW1dlI3HfeB2gj7Yc8XdHh9vkpPyuT nY/ZsFbnvBtiw7GchKKri2gDhRb2QNNDyBnQn5mRFw7CyuFclAksOdV/sdpQnYlYcRQWOUGY HhQ5eqTRZjm9z+qQe/T0HQpmiPTqQcIaG/edgKVTUjITfA7AJMKLQHgp04Vylb+G6jocnQQX JqvvP09whbqrABEBAAHCwWUEGAECAA8CGwwFAlVcpi8FCRmg08MACgkQyx8mb86fmYHNRQ/+ J0OZsBYP4leJvQF8lx9zif+v4ZY/6C9tTcUv/KNAE5leyrD4IKbnV4PnbrVhjq861it/zRQW cFpWQszZyWRwNPWUUz7ejmm9lAwPbr8xWT4qMSA43VKQ7ZCeTQJ4TC8kjqtcbw41SjkjrcTG wF52zFO4bOWyovVAPncvV9eGA/vtnd3xEZXQiSt91kBSqK28yjxAqK/c3G6i7IX2rg6pzgqh hiH3/1qM2M/LSuqAv0Rwrt/k+pZXE+B4Ud42hwmMr0TfhNxG+X7YKvjKC+SjPjqp0CaztQ0H nsDLSLElVROxCd9m8CAUuHplgmR3seYCOrT4jriMFBtKNPtj2EE4DNV4s7k0Zy+6iRQ8G8ng QjsSqYJx8iAR8JRB7Gm2rQOMv8lSRdjva++GT0VLXtHULdlzg8VjDnFZ3lfz5PWEOeIMk7Rj trjv82EZtrhLuLjHRCaG50OOm0hwPSk1J64R8O3HjSLdertmw7eyAYOo4RuWJguYMg5DRnBk WkRwrSuCn7UG+qVWZeKEsFKFOkynOs3pVbcbq1pxbhk3TRWCGRU5JolI4ohy/7JV1TVbjiDI HP/aVnm6NC8of26P40Pg8EdAhajZnHHjA7FrJXsy3cyIGqvg9os4rNkUWmrCfLLsZDHD8FnU mDW4+i+XlNFUPUYMrIKi9joBhu18ssf5i5Q= In-Reply-To: <878qr5ras2.fsf@intel.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 1/20/25 03:21, Jani Nikula wrote: > On Mon, 20 Jan 2025, David Laight wrote: >> On Mon, 20 Jan 2025 12:48:11 +0200 >> Jani Nikula wrote: >> >>> On Sun, 19 Jan 2025, David Laight wrote: >>>> On Sat, 18 Jan 2025 14:58:48 -0800 >>>> Guenter Roeck wrote: >>>> >>>>> On 1/18/25 14:11, David Laight wrote: >>>>>> On Sat, 18 Jan 2025 13:21:39 -0800 >>>>>> Linus Torvalds wrote: >>>>>> >>>>>>> On Sat, 18 Jan 2025 at 09:49, Guenter Roeck wrote: >>>>>>>> >>>>>>>> No idea why the compiler would know that the values are invalid. >>>>>>> >>>>>>> It's not that the compiler knows tat they are invalid, but I bet what >>>>>>> happens is in scale() (and possibly other places that do similar >>>>>>> checks), which does this: >>>>>>> >>>>>>> WARN_ON(source_min > source_max); >>>>>>> ... >>>>>>> source_val = clamp(source_val, source_min, source_max); >>>>>>> >>>>>>> and the compiler notices that the ordering comparison in the first >>>>>>> WARN_ON() is the same as the one in clamp(), so it basically converts >>>>>>> the logic to >>>>>>> >>>>>>> if (source_min > source_max) { >>>>>>> WARN(..); >>>>>>> /* Do the clamp() knowing that source_min > source_max */ >>>>>>> source_val = clamp(source_val, source_min, source_max); >>>>>>> } else { >>>>>>> /* Do the clamp knowing that source_min <= source_max */ >>>>>>> source_val = clamp(source_val, source_min, source_max); >>>>>>> } >>>>>>> >>>>>>> (obviously I dropped the other WARN_ON in the conversion, it wasn't >>>>>>> relevant for this case). >>>>>>> >>>>>>> And now that first clamp() case is done with source_min > source_max, >>>>>>> and it triggers that build error because that's invalid. >>>>>>> >>>>>>> So the condition is not statically true in the *source* code, but in >>>>>>> the "I have moved code around to combine tests" case it now *is* >>>>>>> statically true as far as the compiler is concerned. >>>>>> >>>>>> Well spotted :-) >>>>>> >>>>>> One option would be to move the WARN_ON() below the clamp() and >>>>>> add an OPTIMISER_HIDE_VAR(source_max) between them. >>>>>> >>>>>> Or do something more sensible than the WARN(). >>>>>> Perhaps return target_min on any such errors? >>>>>> >>>>> >>>>> This helps: >>>>> >>>>> - WARN_ON(source_min > source_max); >>>>> - WARN_ON(target_min > target_max); >>>>> - >>>>> /* defensive */ >>>>> source_val = clamp(source_val, source_min, source_max); >>>>> >>>>> + WARN_ON(source_min > source_max); >>>>> + WARN_ON(target_min > target_max); >>>> >>>> That is a 'quick fix' ... >>>> >>>> Much better would be to replace the WARN() with (say): >>>> if (target_min >= target_max) >>>> return target_min; >>>> if (source_min >= source_max) >>>> return target_min + (target_max - target_min)/2; >>>> So that the return values are actually in range (in as much as one is defined). >>>> Note that the >= cpmparisons also remove a divide by zero. >>> >>> I want the loud and early warnings for clear bugs instead of >>> "gracefully" silencing the errors only to be found through debugging >>> user reports. >> >> A user isn't going to notice a WARN() - not until you tell them to look for it. >> In any case even if you output a message you really want to return a 'sane' >> value, who knows what effect a very out of range value is going to have. > > The point is, we'll catch the WARN in CI before it goes out to users. > It isn't going to catch the divide by 0 error, and it obviously doesn't catch the build problem on parisc with gcc 13.x because the CI isn't testing it. How about disabling DRM_XE on architectures where it isn't supported, matching DRM_I915 ? Thanks, Guenter