linux-kernel - Re: [PATCH v2] openrisc: Add cacheinfo support

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Message-ID: <1838f997-f3ec-4df2-bee2-20e1dbe35b3d@gmail.com>
Date: Sun, 23 Mar 2025 00:34:08 +0530
From: Sahil Siddiq <icegambit91@...il.com>
To: Stafford Horne <shorne@...il.com>
Cc: Geert Uytterhoeven <geert@...ux-m68k.org>, jonas@...thpole.se,
 stefan.kristiansson@...nalahti.fi, linux-openrisc@...r.kernel.org,
 linux-kernel@...r.kernel.org, Sahil Siddiq <sahilcdq@...ton.me>
Subject: Re: [PATCH v2] openrisc: Add cacheinfo support

Hi,

On 3/22/25 9:59 PM, Stafford Horne wrote:
> On Sat, Mar 22, 2025 at 07:21:18PM +0530, Sahil Siddiq wrote:
>> On 3/18/25 1:13 PM, Stafford Horne wrote:
>>> On Tue, Mar 18, 2025 at 12:06:30AM +0530, Sahil Siddiq wrote:
>>>> On 3/17/25 1:55 PM, Geert Uytterhoeven wrote:
>>>>> On Sun, 16 Mar 2025 at 07:59, Stafford Horne <shorne@...il.com> wrote:
>>>>> [...]
>>>>>> @@ -176,8 +177,11 @@ void __init paging_init(void)
>>>>>>     	barrier();
>>>>>>     	/* Invalidate instruction caches after code modification */
>>>>>> -	mtspr(SPR_ICBIR, 0x900);
>>>>>> -	mtspr(SPR_ICBIR, 0xa00);
>>>>>> +	upr = mfspr(SPR_UPR);
>>>>>> +	if (upr & SPR_UPR_UP & SPR_UPR_ICP) {
>>>>>> +		mtspr(SPR_ICBIR, 0x900);
>>>>>> +		mtspr(SPR_ICBIR, 0xa00);
>>>>>> +	}
>>>>> Here we could use new utilities such as local_icache_range_inv(0x900,
>>>>> L1_CACHE_BYTES);
>>>>>
>>>>> Or something like local_icache_block_inv(0x900).  This only needs to flush a
>>>>> single block as the code it is invalidating is just 2 instructions 8 bytes:
>>>>>
>>>>>        .org 0x900
>>>>> 	l.j     boot_dtlb_miss_handler
>>>>> 	 l.nop
>>>>>
>>>>>        .org 0xa00
>>>>> 	l.j     boot_itlb_miss_handler
>>>>> 	 l.nop
>>>>
>>>> Given that there'll be generic local_(i|d)cache_range_inv(start, stop) utility
>>>> functions, would it make sense to simply have a macro defined as:
>>>>
>>>> #define local_icache_block_inv(addr) local_icache_range_inv(start, L1_CACHE_BYTES)
>>>>
>>>> instead of having a separate function for invalidating a single cache line? This would
>>>> still use cache_loop() under the hood. The alternative would be to use
>>>> local_icache_range_inv(start, L1_CACHE_BYTES) directly but using the macro might be
>>>> more readable.
>>>
>>> Yes, I think a macro would be fine.  Should we use cache_desc.block_size or
>>> L1_CACHE_BYTES?  It doesn't make much difference as L1_CACHE_BYTES is defined as
>>> 16 bytes which is the minimum block size and using that will always invalidate a
>>> whole block.  It would be good to have a comment explaining why using
>>> L1_CACHE_BYTES is enough.
>>>
>>
>> While working on the patch's v3, I realized I am a bit unclear here. Is the ".org"
>> macro used to set the address at which the instructions are stored in memory? If so,
>> the first two instructions should occupy the memory area 0x900 through 0x907, right?
>> Similarly, the next two instructions will occupy 0xa00-0xa07.
>>
>> Since the two instructions are 256 bytes apart, they shouldn't be cached in the same
>> cache line, right? Maybe one cache line will have 16 bytes starting from 0x900 while
>> another cache line will have 16 bytes starting from 0xa00.
> 
> Yes, to invalidate the cache we will need to do:
> 
> 	local_icache_block_inv(0x900);
> 	local_icache_block_inv(0xa00);
> 
> This will then compile down to the pretty much same as, (but with checks to
> validate the caches exist first):
> 
> 	mtspr(0x900);
> 	mtspr(0xa00);

Ok, this makes sense. I misunderstood the comments in the previous email.

>> If the above is true, I think it'll be better to simply call mtspr() for each address
>> individually.
> 
> Thats right, but I figured the local_icache_block_inv function/macro would be
> more useful other than just this block.
> 

Right, I'll replace this with a macro.

Thanks,
Sahil