| lists.openwall.net | lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC | |
|
Open Source and information security mailing list archives
| ||
|
Message-ID: <69cb064c-2a8e-9421-35b0-4a2273f62365@gmail.com>
Date: Tue, 18 Jun 2019 12:45:20 +0300
From: Dmitry Osipenko <digetx@...il.com>
To: Jon Hunter <jonathanh@...dia.com>,
Laxman Dewangan <ldewangan@...dia.com>,
Vinod Koul <vkoul@...nel.org>,
Thierry Reding <thierry.reding@...il.com>,
Ben Dooks <ben.dooks@...ethink.co.uk>
Cc: dmaengine@...r.kernel.org, linux-tegra@...r.kernel.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1] dmaengine: tegra-apb: Support per-burst residue
granularity
18.06.2019 11:47, Jon Hunter пишет:
>
> On 17/06/2019 13:41, Dmitry Osipenko wrote:
>> 17.06.2019 13:57, Jon Hunter пишет:
>>>
>>> On 14/06/2019 17:44, Dmitry Osipenko wrote:
>>>> 14.06.2019 18:24, Jon Hunter пишет:
>>>>>
>>>>> On 14/06/2019 16:21, Jon Hunter wrote:
>>>>>>
>>>>>> On 13/06/2019 22:08, Dmitry Osipenko wrote:
>>>>>>> Tegra's APB DMA engine updates words counter after each transferred burst
>>>>>>> of data, hence it can report transfer's residual with more fidelity which
>>>>>>> may be required in cases like audio playback. In particular this fixes
>>>>>>> audio stuttering during playback in a chromiuim web browser. The patch is
>>>>>>> based on the original work that was made by Ben Dooks [1]. It was tested
>>>>>>> on Tegra20 and Tegra30 devices.
>>>>>>>
>>>>>>> [1] https://lore.kernel.org/lkml/20190424162348.23692-1-ben.dooks@codethink.co.uk/
>>>>>>>
>>>>>>> Inspired-by: Ben Dooks <ben.dooks@...ethink.co.uk>
>>>>>>> Signed-off-by: Dmitry Osipenko <digetx@...il.com>
>>>>>>> ---
>>>>>>> drivers/dma/tegra20-apb-dma.c | 35 ++++++++++++++++++++++++++++-------
>>>>>>> 1 file changed, 28 insertions(+), 7 deletions(-)
>>>>>>>
>>>>>>> diff --git a/drivers/dma/tegra20-apb-dma.c b/drivers/dma/tegra20-apb-dma.c
>>>>>>> index 79e9593815f1..c5af8f703548 100644
>>>>>>> --- a/drivers/dma/tegra20-apb-dma.c
>>>>>>> +++ b/drivers/dma/tegra20-apb-dma.c
>>>>>>> @@ -797,12 +797,36 @@ static int tegra_dma_terminate_all(struct dma_chan *dc)
>>>>>>> return 0;
>>>>>>> }
>>>>>>>
>>>>>>> +static unsigned int tegra_dma_update_residual(struct tegra_dma_channel *tdc,
>>>>>>> + struct tegra_dma_sg_req *sg_req,
>>>>>>> + struct tegra_dma_desc *dma_desc,
>>>>>>> + unsigned int residual)
>>>>>>> +{
>>>>>>> + unsigned long status, wcount = 0;
>>>>>>> +
>>>>>>> + if (!list_is_first(&sg_req->node, &tdc->pending_sg_req))
>>>>>>> + return residual;
>>>>>>> +
>>>>>>> + if (tdc->tdma->chip_data->support_separate_wcount_reg)
>>>>>>> + wcount = tdc_read(tdc, TEGRA_APBDMA_CHAN_WORD_TRANSFER);
>>>>>>> +
>>>>>>> + status = tdc_read(tdc, TEGRA_APBDMA_CHAN_STATUS);
>>>>>>> +
>>>>>>> + if (!tdc->tdma->chip_data->support_separate_wcount_reg)
>>>>>>> + wcount = status;
>>>>>>> +
>>>>>>> + if (status & TEGRA_APBDMA_STATUS_ISE_EOC)
>>>>>>> + return residual - sg_req->req_len;
>>>>>>> +
>>>>>>> + return residual - get_current_xferred_count(tdc, sg_req, wcount);
>>>>>>> +}
>>>>>>> +
>>>>>>> static enum dma_status tegra_dma_tx_status(struct dma_chan *dc,
>>>>>>> dma_cookie_t cookie, struct dma_tx_state *txstate)
>>>>>>> {
>>>>>>> struct tegra_dma_channel *tdc = to_tegra_dma_chan(dc);
>>>>>>> + struct tegra_dma_sg_req *sg_req = NULL;
>>>>>>> struct tegra_dma_desc *dma_desc;
>>>>>>> - struct tegra_dma_sg_req *sg_req;
>>>>>>> enum dma_status ret;
>>>>>>> unsigned long flags;
>>>>>>> unsigned int residual;
>>>>>>> @@ -838,6 +862,8 @@ static enum dma_status tegra_dma_tx_status(struct dma_chan *dc,
>>>>>>> residual = dma_desc->bytes_requested -
>>>>>>> (dma_desc->bytes_transferred %
>>>>>>> dma_desc->bytes_requested);
>>>>>>> + residual = tegra_dma_update_residual(tdc, sg_req, dma_desc,
>>>>>>> + residual);
>>>>>>
>>>>>> I had a quick look at this, I am not sure that we want to call
>>>>>> tegra_dma_update_residual() here for cases where the dma_desc is on the
>>>>>> free_dma_desc list. In fact, couldn't this be simplified a bit for case
>>>>>> where the dma_desc is on the free list? In that case I believe that the
>>>>>> residual should always be 0.
>>>>>
>>>>> Actually, no, it could be non-zero in the case the transfer is aborted.
>>>>
>>>> Looks like everything should be fine as-is.
>>>
>>> I am still not sure we want to call this for the case where dma_desc is
>>> on the free list.
>>
>> You're right! It's a bug there! The sg_req=NULL if dma_desc is on the free list, hence
>> it will result in a NULL dereference. I'll fix it in v2 and will avoid the offending
>> call, like you're suggesting.
>>
>>>> BTW, it's a bit hard to believe that there is any real benefit from the
>>>> free_dma_desc list at all, maybe worth to just remove it?
>>>
>>> I think you need to elaborate a bit more here. I am not a massive fan of
>>> this driver, but I am also not in the mood for changing unless there is
>>> a good reason.
>>
>> It looks like the whole point of the free list is to have a cache of preallocated
>> dma_desc's, but dma_desc allocation and initialization doesn't cost anything in
>> comparison to the free list because memory is allocated from a SLAB cache and then the
>> initialization will happen on CPU's cache.
>>
>> So the free list is quite pointless in terms of optimization. Moreover what if driver
>> allocates a lot of dma_desc's and uses them just once? Looks like it will be quite a
>> lot of wasted memory on the free list.
>
> Yes indeed and for the ADMA we allocate and free on-demand as you are
> suggesting. I don't know why it was done like this, but to make the
> change it would be good to get some data about how much memory it is
> consuming to see if it is actually worth it.
Yeah, that's something to check in the future.
Powered by blists - more mailing lists