tidy'ing up cz_hwmgr.c

[Deucher, Alexander]

IIRC, zero sized arrays are not technically allowed in C, although gcc allows them.  As I said, some versions of gcc worked, others didn't.  I'm not sure why.  Also, my example was slightly wrong.  atombios.h uses arrays of size 1, not 0.  So my example should look like:

struct table {
   uint16_t size;
   struct element elements[1];
};

atombios.h uses [1] since I don't think [0] is portable.  The same indexing issue applies to [1].

Alex

From: amd-gfx [mailto:amd-gfx-bounces at lists.freedesktop.org] On Behalf Of StDenis, Tom
Sent: Thursday, August 18, 2016 11:40 AM
To: Alex Deucher
Cc: Christian König; amd-gfx list
Subject: Re: tidy'ing up cz_hwmgr.c


It had to be something more complicated because this demo program


#include <stdio.h>
#include <stdlib.h>

struct one {
char *foo;
int bar[0];
};

struct two {
char *foo;
int bar[1];
};

int main(void)
{
struct one *a = calloc(1, sizeof(struct one) + 4 * sizeof(int));
struct two *b = calloc(1, sizeof(struct two) + 3 * sizeof(int));
int x;

printf("a == %p\n", a);
for (x = 0; x < 4; x++)
printf("&a.bar[%d] = %p\n", x, &a->bar[x]);

printf("b == %p\n", b);
for (x = 0; x < 4; x++)
printf("&b.bar[%d] = %p\n", x, &b->bar[x]);

return 0;
}


produces this output


tom at fx8:~$ gcc test.c -o test
tom at fx8:~$ ./test
a == 0x1fd4010
&a.bar[0] = 0x1fd4018
&a.bar[1] = 0x1fd401c
&a.bar[2] = 0x1fd4020
&a.bar[3] = 0x1fd4024
b == 0x1fd4030
&b.bar[0] = 0x1fd4038
&b.bar[1] = 0x1fd403c
&b.bar[2] = 0x1fd4040
&b.bar[3] = 0x1fd4044

Which is exactly what you'd expect.  I'm not strongly advocating we change the PP code just noting it's not really clear that it's correct from a first reading and in theory would be better with [0].

Tom
________________________________
From: Alex Deucher <alexdeucher at gmail.com<mailto:alexdeucher at gmail.com>>
Sent: Thursday, August 18, 2016 11:33
To: StDenis, Tom
Cc: Christian König; amd-gfx list
Subject: Re: tidy'ing up cz_hwmgr.c

The problem we ran into was when we had a struct like this:
struct table {
   uint16_t size;
   struct element elements[0];
};

and then we would try and index the array:
for (i = 0; i < table->size; i++) {
  element = &table->elements[i];
}
element ended up off in the weeds.  The only thing that seems to make some versions of gcc happy was pointer arithmetic.  E.g.,

element = (struct element *)((char *)&table->elements[0] + (sizeof(struct element) * i));
Alex

On Thu, Aug 18, 2016 at 11:21 AM, StDenis, Tom <Tom.StDenis at amd.com<mailto:Tom.StDenis at amd.com>> wrote:

Any modern GCC should support [0] at the tail of a struct.  This came up because when I was reading the code I saw they allocated 7 slots (plus the size of the struct) but then fill 8 slots.  It's just weird [😊]



Using [0] in the struct and allocating for 8 entries makes more sense and is clearer to read.



Tom

________________________________
From: Christian König <deathsimple at vodafone.de<mailto:deathsimple at vodafone.de>>
Sent: Thursday, August 18, 2016 11:17
To: StDenis, Tom; amd-gfx list
Subject: Re: tidy'ing up cz_hwmgr.c

Has a [1] array at the tail which is then kzalloc'ed with N-1 entries.  Shouldn't that just be a [0] with N entries allocated for clarity?
Actually the starting address of a dynamic array should be manually calculated instead of using [1] or [0].

We had tons of problems with that because some gcc versions get this wrong and the atombios code used this as well.

Alex how did we resolved such issues?

Regards,
Christian.

Am 18.08.2016 um 16:26 schrieb StDenis, Tom:

Tidying up cz_hwmgr.c I noted a couple of things but first is


static bool cz_dpm_check_smu_features(struct pp_hwmgr *hwmgr,
unsigned long check_feature);

Which will return "true" if the smu call fails or the feature is set.

The structure

struct phm_clock_voltage_dependency_table;

Has a [1] array at the tail which is then kzalloc'ed with N-1 entries.  Shouldn't that just be a [0] with N entries allocated for clarity?

Tom




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