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Clarified documentation in few places to mention textures need to use LINEAR layout when accessed from the host

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Adam Sawicki 2020-05-08 18:43:25 +02:00
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docs/html/usage_patterns.html
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<title>Vulkan Memory Allocator: Recommended usage patterns</title>
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@ -93,7 +93,7 @@ Immutable resources</h2>
Dynamic resources</h2>
<p><b>When:</b> Any resources that change frequently (aka "dynamic"), e.g. every frame or every draw call, written on CPU, read on GPU.</p>
<p><b>What to do:</b> Create them using <a class="el" href="vk__mem__alloc_8h.html#aa5846affa1e9da3800e3e78fae2305cca9066b52c5a7079bb74a69aaf8b92ff67">VMA_MEMORY_USAGE_CPU_TO_GPU</a>. You can map it and write to it directly on CPU, as well as read from it on GPU.</p>
<p>This is a more complex situation. Different solutions are possible, and the best one depends on specific GPU type, but you can use this simple approach for the start. Prefer to write to such resource sequentially (e.g. using <code>memcpy</code>). Don't perform random access or any reads from it on CPU, as it may be very slow.</p>
<p>This is a more complex situation. Different solutions are possible, and the best one depends on specific GPU type, but you can use this simple approach for the start. Prefer to write to such resource sequentially (e.g. using <code>memcpy</code>). Don't perform random access or any reads from it on CPU, as it may be very slow. Also note that textures written directly from the host through a mapped pointer need to be in LINEAR not OPTIMAL layout.</p>
<h2><a class="anchor" id="usage_patterns_readback"></a>
Readback</h2>
<p><b>When:</b> Resources that contain data written by GPU that you want to read back on CPU, e.g. results of some computations.</p>
@ -108,9 +108,9 @@ Detecting integrated graphics</h2>
Direct access versus transfer</h2>
<p>For resources that you frequently write on CPU and read on GPU, many solutions are possible:</p>
<ol type="1">
<li>Create one copy in video memory using <a class="el" href="vk__mem__alloc_8h.html#aa5846affa1e9da3800e3e78fae2305ccac6b5dc1432d88647aa4cd456246eadf7">VMA_MEMORY_USAGE_GPU_ONLY</a>, second copy in system memory using <a class="el" href="vk__mem__alloc_8h.html#aa5846affa1e9da3800e3e78fae2305cca40bdf4cddeffeb12f43d45ca1286e0a5">VMA_MEMORY_USAGE_CPU_ONLY</a> and submit explicit tranfer each time.</li>
<li>Create just single copy using <a class="el" href="vk__mem__alloc_8h.html#aa5846affa1e9da3800e3e78fae2305cca9066b52c5a7079bb74a69aaf8b92ff67">VMA_MEMORY_USAGE_CPU_TO_GPU</a>, map it and fill it on CPU, read it directly on GPU.</li>
<li>Create just single copy using <a class="el" href="vk__mem__alloc_8h.html#aa5846affa1e9da3800e3e78fae2305cca40bdf4cddeffeb12f43d45ca1286e0a5">VMA_MEMORY_USAGE_CPU_ONLY</a>, map it and fill it on CPU, read it directly on GPU.</li>
<li>Create one copy in video memory using <a class="el" href="vk__mem__alloc_8h.html#aa5846affa1e9da3800e3e78fae2305ccac6b5dc1432d88647aa4cd456246eadf7">VMA_MEMORY_USAGE_GPU_ONLY</a>, second copy in system memory using <a class="el" href="vk__mem__alloc_8h.html#aa5846affa1e9da3800e3e78fae2305cca40bdf4cddeffeb12f43d45ca1286e0a5">VMA_MEMORY_USAGE_CPU_ONLY</a> and submit explicit transfer each time.</li>
<li>Create just a single copy using <a class="el" href="vk__mem__alloc_8h.html#aa5846affa1e9da3800e3e78fae2305cca9066b52c5a7079bb74a69aaf8b92ff67">VMA_MEMORY_USAGE_CPU_TO_GPU</a>, map it and fill it on CPU, read it directly on GPU.</li>
<li>Create just a single copy using <a class="el" href="vk__mem__alloc_8h.html#aa5846affa1e9da3800e3e78fae2305cca40bdf4cddeffeb12f43d45ca1286e0a5">VMA_MEMORY_USAGE_CPU_ONLY</a>, map it and fill it on CPU, read it directly on GPU.</li>
</ol>
<p>Which solution is the most efficient depends on your resource and especially on the GPU. It is best to measure it and then make the decision. Some general recommendations:</p>
<ul>
@ -124,6 +124,7 @@ Direct access versus transfer</h2>
<li>Create just single copy using <a class="el" href="vk__mem__alloc_8h.html#aa5846affa1e9da3800e3e78fae2305cca7b586d2fdaf82a463b58f581ed72be27">VMA_MEMORY_USAGE_GPU_TO_CPU</a>, write to it directly on GPU, map it and read it on CPU.</li>
</ol>
<p>You should take some measurements to decide which option is faster in case of your specific resource.</p>
<p>Note that textures accessed directly from the host through a mapped pointer need to be in LINEAR layout, which may slow down their usage on the device. Textures accessed only by the device and transfer operations can use OPTIMAL layout.</p>
<p>If you don't want to specialize your code for specific types of GPUs, you can still make an simple optimization for cases when your resource ends up in mappable memory to use it directly in this case instead of creating CPU-side staging copy. For details see <a class="el" href="memory_mapping.html#memory_mapping_finding_if_memory_mappable">Finding out if memory is mappable</a>. </p>
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@ -131,7 +132,7 @@ Direct access versus transfer</h2>
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