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Open-Source AMD OpenGL Driver Drops Support For Smart Access Memory / ReBAR

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  • #11
    To clarify there are a lot of variants of this:
    1. Older platforms that support >4G decoding. This provides an additional MMIO region above 4G that can be used for MMIO. The GPU kernel driver can resize the BARs at driver load time if there is enough MMIO space available. The BIOS does not resize the GPU BAR in this case, the GPU kernel driver does. Some of these platforms seem to have bugs where they set incorrect CPU cache mappings on parts the extended MMIO space. This feature was mainly implemented for mining so there would enough MMIO space on the platform for lots of devices with large BARs like GPUs.
    2. Newer platforms that support ReBAR or SAM. This provides an additional MMIO region above 4G that can be used for MMIO and the BIOS resizes the GPU BAR when it enumerates the devices on the platform. In this case, the GPU kernel driver doesn't resize the BAR because the BIOS has already done it. There seem to be fewer BIOS bugs in this case because this feature was specifically targetting GPUs so it's better verified by platform vendors.

    Regardless of whether you are using 1 or 2 above, the GPU kernel driver has direct access to the entire amount of VRAM via the PCI BAR. As such, if a UMD requests CPU access to a GPU VRAM buffer, we don't have to worry about migrating to system memory if there is contention for due to having a smaller VRAM BAR. This is always taken advantage of regardless of what the UMD decides to do.

    Finally, in addition to all of this, the UMDs also made decisions about certain behavior. These particular behaviors are what the article is about.

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    • #12
      On opengl sre not only ole games, but all CAD software (also new), so stop with slowly droping of Opengl. Is not true, that vulkan is more important than opengl. Decisions made childs or what ?

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      • #13
        Originally posted by Atirage21 View Post
        On opengl sre not only ole games, but all CAD software (also new), so stop with slowly droping of Opengl. Is not true, that vulkan is more important than opengl. Decisions made childs or what ?
        I'll have a go at translating this:

        "On OpenGL are not only old games, but also all CAD software (including new ones). So, please stop slowly deprecating OpenGL features. It's not true that Vulkan is more important than OpenGL."

        Based on Marek and agd5f's posts, I'd say you needn't worry about this change. It's not going to break your software and is more likely to make it run faster than slower. Sounds like it could even avoid some bugs, too.

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        • #14
          This article is somewhat misleading on what exactly is being removed, I recommend clarifying it.

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          • #15
            Originally posted by pinguinpc View Post
            In my case rebar give more performance or personally that seems in dead rising 2

            Actually my system use rebar and run most titles very good in my little UHD 630 CFL-GT2

            ...
            You apparently missed the entire article including the headline... This is strictly about RadeonSI (AMD OpenGL) which has nothing to do with your Intel GPU hardware.

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            • #16
              Originally posted by pinguinpc View Post
              In my case rebar give more performance or personally that seems in dead rising 2
              ...
              Yeah this is also incredibly short sited in my view. Even on Windows with DX, ReBAR did not globally improve performance of every game, it was on a game by game basis. In fact most of the work that AMD did with ReBAR on Windows wasn't the feature itself, but rather creating a giant lookup table to figure out for which games it should be enabled and for which ones not (because in some games it worsened performance).

              They should at least leave the functionality in there and leave it things like SteamOS to figure out for which games it should be enabled

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              • #17
                AMD Smart Access Memory is just PCIe Resizable BAR (ReBAR) but branded for AMD Ryzen + AMD Radeon system combinations.
                Well, here's what bridgman said in 2021 about that, in these forums no less.

                SAM = marketing term for driver code that takes advantage of having a BAR that covers the entire VRAM and improves performance by doing fewer and/or faster copies.

                note that resizeable BAR is a kernel driver feature (which has been in place for a year or more) while "taking advantage of the larger BAR" mostly happens in the userspace drivers (radeonsi, possibly radv). That's one of the reasons we keep SAM and resizeable BAR separate when talking about them.​

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                • #18
                  Originally posted by mdedetrich View Post

                  Yeah this is also incredibly short sited in my view. Even on Windows with DX, ReBAR did not globally improve performance of every game, it was on a game by game basis. In fact most of the work that AMD did with ReBAR on Windows wasn't the feature itself, but rather creating a giant lookup table to figure out for which games it should be enabled and for which ones not (because in some games it worsened performance).

                  They should at least leave the functionality in there and leave it things like SteamOS to figure out for which games it should be enabled
                  Have you read the statements made by Marek and Alex just some posts above?

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                  • #19
                    Originally posted by Hibbelharry View Post

                    Have you read the statements made by Marek and Alex just some posts above?
                    Only after I made the post

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                    • #20
                      Originally posted by mbriar View Post

                      Did you ever get any measurable improvement from it in OpenGL games?
                      I haven't seen anything earthshattering yet. But the games I tried it with were older and probably not taxing the GPU that much anyway.
                      Last edited by ms178; 14 March 2023, 02:53 PM.

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