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SubjectRe: [git pull] drm for v4.15
On Fri, Nov 17, 2017 at 10:14 AM, Christian König
<christian.koenig@amd.com> wrote:
>
> Taking an example from the AMD headers why this automation is more tricky
> than it sounds in the first place: Look at the
> mmVM_CONTEXT*_PAGE_TABLE_BASE_ADDR registers for example.
>
> Register 0-7 are consecutive and so could be perfectly addressable with an
> index, but register 8-15 aren't and so we always end with logic like if(i<8)
> ... else ....
>
> The rational from the hardware guys is obvious that they initially had only
> 8 and on a later hardware generation extended that to 16 registers.

Heh. I don't disagree, but at the same time, that case is actually a
wonderful example.

Let's take the gmc_6_0 case, because it shows your irregularity, but
it also shows another horrid example of nasty nasty automation:

mmVM_CONTEXT0_PAGE_TABLE_BASE_ADDR 0x054F
mmVM_CONTEXT10_PAGE_TABLE_BASE_ADDR 0x0510
mmVM_CONTEXT11_PAGE_TABLE_BASE_ADDR 0x0511
mmVM_CONTEXT12_PAGE_TABLE_BASE_ADDR 0x0512
mmVM_CONTEXT13_PAGE_TABLE_BASE_ADDR 0x0513
mmVM_CONTEXT14_PAGE_TABLE_BASE_ADDR 0x0514
mmVM_CONTEXT15_PAGE_TABLE_BASE_ADDR 0x0515
mmVM_CONTEXT1_PAGE_TABLE_BASE_ADDR 0x0550
mmVM_CONTEXT2_PAGE_TABLE_BASE_ADDR 0x0551
mmVM_CONTEXT3_PAGE_TABLE_BASE_ADDR 0x0552
mmVM_CONTEXT4_PAGE_TABLE_BASE_ADDR 0x0553
mmVM_CONTEXT5_PAGE_TABLE_BASE_ADDR 0x0554
mmVM_CONTEXT6_PAGE_TABLE_BASE_ADDR 0x0555
mmVM_CONTEXT7_PAGE_TABLE_BASE_ADDR 0x0556
mmVM_CONTEXT8_PAGE_TABLE_BASE_ADDR 0x050E
mmVM_CONTEXT9_PAGE_TABLE_BASE_ADDR 0x050F

Oops. Those were clearly sorted automatically, and in entirely the wrong way.

So automation has _really_ done something inexcusably stupid, and made
the end result completely illegible in the process.

And yes, you'd be right that it's discontiguous at 8, but it's still
arithmetic, ie you could easily have

#define mmVM_PAGE_TABLE_BASE_ADDR(ctx) \
((ctx)+0x054f-((ctx) & 8)*9-((ctx)&8)/8)

and if "ctx" is a constant, then the end result is trivially a
constant and can be used as such. And if it isn't, it's still a much
cheaper operation than an "if" or "switch ()" statement (it's just a
bitmask and two shifts).

Now, seeing those patterns is likely not something that automation
should do (although it's definitely possible - superoptimizers do that
all the time), but automation could still *verify* the patterns once a
human has made them up.

And it's quite possible that it would be a good idea to encode that
pattern even in the original source code. In fact, it may *be* there
somewhere (not as that arithmetic expression, but as the reverse
decode logic, obviously).

Linus

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