linux-kernel - Re: [PATCH 09/18] soc: qcom: ipa: GSI transactions

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Date:   Wed, 15 May 2019 07:25:58 -0500
From:   Alex Elder <elder@...aro.org>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     David Miller <davem@...emloft.net>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Ilias Apalodimas <ilias.apalodimas@...aro.org>,
        syadagir@...eaurora.org, mjavid@...eaurora.org,
        evgreen@...omium.org, benchan@...gle.com, ejcaruso@...gle.com,
        abhishek.esse@...il.com,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 09/18] soc: qcom: ipa: GSI transactions

On 5/15/19 2:34 AM, Arnd Bergmann wrote:
>> +static void gsi_trans_tre_fill(struct gsi_tre *dest_tre, dma_addr_t addr,
>> +                              u32 len, bool last_tre, bool bei,
>> +                              enum ipa_cmd_opcode opcode)
>> +{
>> +       struct gsi_tre tre;
>> +
>> +       tre.addr = cpu_to_le64(addr);
>> +       tre.len_opcode = gsi_tre_len_opcode(opcode, len);
>> +       tre.reserved = 0;
>> +       tre.flags = gsi_tre_flags(last_tre, bei, opcode);
>> +
>> +       *dest_tre = tre;        /* Write TRE as a single (16-byte) unit */
>> +}
> 
> Have you checked that the atomic write is actually what happens here,
> but looking at the compiler output? You might need to add a 'volatile'
> qualifier to the dest_tre argument so the temporary structure doesn't
> get optimized away here.

No, and I really should have checked, since I'm assuming that's
what will happen.  I will check, and may well add the volatile
regardless.

>> +/* Cancel a channel's pending transactions */
>> +void gsi_channel_trans_cancel_pending(struct gsi_channel *channel)
>> +{
>> +       struct gsi_trans_info *trans_info = &channel->trans_info;
>> +       u32 evt_ring_id = channel->evt_ring_id;
>> +       struct gsi *gsi = channel->gsi;
>> +       struct gsi_evt_ring *evt_ring;
>> +       struct gsi_trans *trans;
>> +       unsigned long flags;
>> +
>> +       evt_ring = &gsi->evt_ring[evt_ring_id];
>> +
>> +       spin_lock_irqsave(&evt_ring->ring.spinlock, flags);
>> +
>> +       list_for_each_entry(trans, &trans_info->pending, links)
>> +               trans->result = -ECANCELED;
>> +
>> +       list_splice_tail_init(&trans_info->pending, &trans_info->complete);
>> +
>> +       spin_unlock_irqrestore(&evt_ring->ring.spinlock, flags);
>> +
>> +       spin_lock_irqsave(&gsi->spinlock, flags);
>> +
>> +       if (gsi->event_enable_bitmap & BIT(evt_ring_id))
>> +               gsi_event_handle(gsi, evt_ring_id);
>> +
>> +       spin_unlock_irqrestore(&gsi->spinlock, flags);
>> +}
> 
> That is a lot of irqsave()/irqrestore() operations. Do you actually call
> all of these functions from hardirq context?

The transaction list is definitely updated in IRQ context,
but I think it is no longer updated in hardirq context (the
softirq was a recent change).  This particular function is
definitely not called in a hardirq context, so I can remove
the irqsave/irqrestore.

I'll survey my spinlock use throughout the driver and will
remove any irqsave/irqrestore used in non-hardirq contexts.

Thanks.

					-Alex


>       Arnd
>