Re: [PATCH] drm/i915: Consider HW CSB write pointer before resetting the sw read pointer

[Michel Thierry <michel.thierry@intel.com>]

On 9/25/2015 4:44 PM, Mika Kuoppala wrote:
> Michel Thierry <michel.thierry@intel.com> writes:
>> -	ring->next_context_status_buffer = 0;
>> +
>> +	/*
>> +	 * Instead of resetting the Context Status Buffer (CSB) read pointer to
>> +	 * zero, we need to read the write pointer from hardware and use its
>> +	 * value because "this register is power context save restored".
>> +	 * Effectively, these states have been observed:
>> +	 *
>> +	 *      | Suspend-to-idle (freeze) | Suspend-to-RAM (mem) |
>> +	 * BDW  | CSB regs not reset       | CSB regs reset       |
>> +	 * CHT  | CSB regs not reset       | CSB regs not reset   |
>> +	 */
>> +	next_context_status_buffer_hw = I915_READ(RING_CONTEXT_STATUS_PTR(ring)) & 0x07;
>> +
>> +	/*
>> +	 * When the CSB registers are reset (also after power-up / gpu reset),
>> +	 * CSB write pointer is set to all 1's, which is not valid, use 0 in
>> +	 * this special case.
>> +	 */
>> +	if (next_context_status_buffer_hw == 0x7)
>> +		next_context_status_buffer_hw = 0;
>
>
> If hardware has been reset and we have b111 here, I assume
> the first write will be at index zero.
>
> If we look at the interrupt code there is while (read_pointer <
> write_pointer). Initializing next status buffer to zero
> would mean that you miss the first write to csb[0].
>
> If b111 is found, I think the correct value is 5.

Correct, it needs to be set to 5. Luckily, csb[0] would always be 
Idle-to-Active, which we ignore.

>
> -Mika
>
>

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