Subject: Re: [PATCH v2 5/5] crypto: talitos: Add software backlog queue handling

From: Horia Geantă <[email-masked]>
Date: Thu, 5 Mar 2015 11:35:23 +0200

On 3/4/2015 2:23 AM, Kim Phillips wrote:
> On Tue,  3 Mar 2015 08:21:37 -0500
> Martin Hicks <[email-masked]> wrote:
> 
>> @@ -1170,6 +1237,8 @@ static struct talitos_edesc *talitos_edesc_alloc(struct device *dev,
>>  						     edesc->dma_len,
>>  						     DMA_BIDIRECTIONAL);
>>  	edesc->req.desc = &edesc->desc;
>> +	/* A copy of the crypto_async_request to use the crypto_queue backlog */
>> +	memcpy(&edesc->req.base, areq, sizeof(struct crypto_async_request));
> 
> this seems backward, or, at least can be done more efficiently IMO:
> talitos_cra_init should set the tfm's reqsize so the rest of
> the driver can wholly embed its talitos_edesc (which should also
> wholly encapsulate its talitos_request (i.e., not via a pointer))
> into the crypto API's request handle allocation.  This
> would absorb and eliminate the talitos_edesc kmalloc and frees, the
> above memcpy, and replace the container_of after the
> crypto_dequeue_request with an offset_of, right?
> 
> When scatter-gather buffers are needed, we can assume a slower-path
> and make them do their own allocations, since their sizes vary
> depending on each request.  Of course, a pointer to those
> allocations would need to be retained somewhere in the request
> handle.

Unfortunately talitos_edesc structure size is most of the times
variable. Its exact size can only be established at "request time", and
not at "tfm init time".

Fixed size would be sizeof(talitos_edesc).
Below are factors that influence the variable part, i.e. link_tbl in
talitos_edesc:
- whether any assoc / src / dst data is scattered
- icv_stashing (case when ICV checking is done in SW)

Still we'd be better with:
-crypto API allocates request + request context (i.e.
sizeof(talitos_edesc) + any alignment required)
-talitos driver allocates variable part of talitos_edesc (if needed)

instead of:
-crypto API allocates request
-talitos driver allocates talitos_edesc (fixed + variable)
-memcopy of the req.base (crypto_async_request) into talitos_edesc

both in terms of performance and readability.

At first look, the driver wouldn't change that much:
-talitos_cra_init() callback would have to set tfm.reqsize to
sizeof(talitos_edesc) + padding and also add the CRYPTO_TFM_REQ_DMA
indication in tfm.crt_flags
-talitos_edesc_alloc() logic would be pretty much the same, but would
allocate memory only for the link_tbl

I'm willing to do these changes if needed.

> 
> Only potential problem is getting the crypto API to set the GFP_DMA
> flag in the allocation request, but presumably a
> CRYPTO_TFM_REQ_DMA crt_flag can be made to handle that.

Right. And this flag would apply only to request __ctx[].

Herbert, would this be an acceptable addition to crypto API?

Thanks,
Horia