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SubjectRe: [PATCH/RFC] SPI: add async message handing library to David Brownell's core
Stephen Street wrote:

>On Tue, 2005-12-13 at 17:06 +0300, Vitaly Wool wrote:
>>This thingie hasn't been thoroughly tested yet, but it's lightweight
>>and easy to understand so I don't think solving the problems that
>>may arise will take long. Though I haven't actually done that yet,
>>I'm sure that Stephen's PXA SSP driver will become easier to understand
>>and less in footprint and will work faster when it's rewritten using
>>this library. (Yes, I do expect performance improvement here as the
>>current implementation schedules multiple tasklets, so
>>scheduling penalty is high.)
>Is this really true? Is tasklet scheduling "harder" than kernal thread
>scheduling? A close look at my PXA SSP SPI implementation will reveal
>that my design is nearly lock-less and callable from any execution
>context (i.e. interrupt context).
It's harder in your case because the tasklet is created each time it's
scheduled again, as far as I see it in your impleemntation.
Each SPI controller thread is created only once so it's more lightweight
than what you do.

>>+ * spi_queue - (internal) queue the message to be processed asynchronously
>>+ * @spi: SPI device to perform transfer to/from
>>+ * @msg: message to be sent
>>+ * Description:
>>+ * This function queues the message to SPI controller's queue.
>>+ */
>>+static int spi_queue(struct spi_device *spi, struct spi_message *msg)
>>+ struct threaded_async_data *td = spi->master->context;
>>+ int rc = 0;
>>+ if (!td) {
>>+ rc = -EINVAL;
>>+ goto out;
>>+ }
>>+ msg->spi = spi;
>>+ down(&td->lock);
>>+ list_add_tail(&msg->queue, &td->msgs);
>>+ dev_dbg(spi->dev.parent, "message has been queued\n");
>>+ up(&td->lock);
>>+ wake_up_interruptible(&td->wq);
>>+ return rc;
>This can not be invoke this from "interrupt context" which is a
>requirement for my SPI devices (CS8415A, CS8405A, CS4341).
Okay, not a major issue though. Change mutexes to
spin_lock_irq/spin_unlock_irq and it's callable from an interrupt
context, right?

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