Toshiba e740 GAPI blit

[Digby]

[sponge]

[Digby]

I don't hate the e740. I actually have one sitting here on my desk. Except for the slow writes to video memory, it's no worse than any of the other XScale-based devices.

I can't speak for Dan or Randall, but if I were designing a cutting edge game that was going to take all of the CPU horsepower available to get my content on the screen at an acceptable frame rate, I've got the following choices to get that game on an e740:

1) Spend time authoring less detailed content (low poly count models, fewer textures, maybe lower quality audio, etc.) in order to save CPU cycles that could be then used on the slower writes to display memory.

2) Decrease the screen size so that the back buffer to front buffer copy operation is moving fewer pixels each frame.

3) Get some info from either Toshiba or ATI on how best to use their hardware. Maybe there is some trick in accessing the video memory on the ATI chip that might help. Who knows? No offense aimed at other people in this forum, but I'm not going to risk releasing a commercial game based on what someone has managed to figure out of the functions in the ATI Imageon DLL. That is just begging for a support nightmare.

4) The final option is do nothing special for this one device and just let the app run slower on the e740. I'd make a demo available of the game so that people would know what they are getting themselves into before they bought it. Maybe 8 FPS is playable for some people with e740s? Who knows? At least I have the chance at making *some* revenue from e740 owners. I'd probably add some sort of Options dialog that would turn off certain features in order to update the frame rate a bit, but that's it. I'm not going to ask my art staff to re-author dumbed down content.

Anyway, that's my take on it. Like I said, it's not a horrible device on the whole, just the video memory bandwidth is subpar.

One thing people fail to realize is that OEMs like Toshiba use graphics chips like the Imageon not for their performance, but rather for their ability to decrease the number of cycles used by the CPU and thereby reduce power consumption.

[Sergey Chaban]

[Sergey Chaban]

Let me clarify some points. While it's true that ATI chip supports video acceleration, even video app would suffer from the same issue - access to video memory is very slow. Imageon supports one h/w overlay with different color modes, PocketMVP uses YUV420 color mode, actual yuv data is written into offscreen surface, then BitBlt function performs colorspace conversion. That last conversion stage this is what is accelerated by the chip. While this helps with video decoding a bit, it seems that actual speedup is not that great as it could be - writes to vmem are still slow and you have to get yuv data to vmem somehow.
Exactly the same sequence works for RGB data, only overlay color type will be different. That is, if chip has support for 8-bit paletted RGB data, then speedup would be the same as for YUV420.
In either case - blits are fast while vmem writes are slow, so while speedup from using ATI API exists it's pretty small.
Another way to speedup vmem writes is to use some specific write order, which is faster due to chip specific write-coalescing or caching or whatever it is. Again speedup is small.
Another situation where ATI API may help is non-fullscreen blits, since it's possible to use straight memcpy to the surface rather than copying buffer line-by-line.
The biggest problem is that accessing active screen buffer (as returned by GAPI) is even slower than accessing an off-screen surface. BTW offscreen memory is available thru the GAPI pointer but only for reading (and it's also useless without ATI API).
Well, I think I'm repeating myself, but that's just to summarize so don't flame me for that

For what I'm doing it's relatively painless to add ATI support (it's about 50 LOC anyway), we don't use GAPI directly and we already have support for several h/w interfaces. For others that may be too much of a hassle. BTW, we already contacted ATI in order to obtain official SDK, but I honestly don't understand their reasons to hide the specs/SDK. I think e740 is not going to be the last and only device with ATI chip, so it may be worthwhile to support it. Anyhow, it's of course everyone's own bussiness whether to support something or not.


SC

[Guest]

[Robotbeat]

[randall]

yeah. try:

[HKEY_LOCAL_MACHINE\HARDWARE\ATI\SECRET_SDK_STUFF]
REMOVE_BOTTLENECK=1
RUN_FULLSPEED=1

[another guest]

oh look! A constructive and encouraging post by randall...

[brendan]

Unforchantly most registry keys that "fix" MS issues etc dont exist to start with, so you a, have to know what the key should be called, b, where it goes, and c, it's value. I'd assume ATI is no diff...


-B

[Sergey Chaban]

[James S]

[another guest]

[Robotbeat]

[sponge]