Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Subject: Re: [PATCH 07/16] spi: bcm63xx-hsspi: Add polling mode support
To:     Mark Brown <broonie@kernel.org>
Cc:     Linux SPI List <linux-spi@vger.kernel.org>,
        Broadcom Kernel List <bcm-kernel-feedback-list@broadcom.com>,
        anand.gore@broadcom.com, tomer.yacoby@broadcom.com,
        dan.beygelman@broadcom.com, joel.peshkin@broadcom.com,
        f.fainelli@gmail.com, jonas.gorski@gmail.com,
        kursad.oney@broadcom.com, dregan@mail.com,
        linux-kernel@vger.kernel.org
References: <20230106200809.330769-1-william.zhang@broadcom.com>
 <20230106200809.330769-8-william.zhang@broadcom.com>
 <Y7iW38Fsj0nIewDm@sirena.org.uk>
 <ec84b84b-41be-32ad-2e76-afac59a621d0@broadcom.com>
 <Y7xloRuHk5BHSOCb@sirena.org.uk>
 <ca22c9af-34bc-e857-881c-263f70a405e8@broadcom.com>
 <Y73rl8feUOnChWKF@sirena.org.uk>
From:   William Zhang <william.zhang@broadcom.com>
Message-ID: <dee51504-ccd2-3c27-ffc8-2e392f2bd781@broadcom.com>
Date:   Wed, 11 Jan 2023 12:13:43 -0800
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101
 Thunderbird/52.4.0
MIME-Version: 1.0
In-Reply-To: <Y73rl8feUOnChWKF@sirena.org.uk>
Content-Type: multipart/signed; protocol="application/pkcs7-signature"; micalg=sha-256;
        boundary="000000000000342f6e05f202a34b"
Precedence: bulk

--000000000000342f6e05f202a34b
Content-Type: text/plain; charset=windows-1252; format=flowed
Content-Language: en-US
Content-Transfer-Encoding: 7bit


On 01/10/2023 02:49 PM, Mark Brown wrote:
> On Mon, Jan 09, 2023 at 12:10:30PM -0800, William Zhang wrote:
>> On 01/09/2023 11:06 AM, Mark Brown wrote:
> 
>>> You can put whatever logic is needed in the code - for something like
>>> this an architecture based define isn't ideal but is probably good
>>> enough if need be (though I'd not be surprised if it turned out that
>>> there was also some performance benefit for the MIPS systems too, at
>>> least for smaller transfers).
> 
>> I just don't know what other logic I can put in the driver to select
>> interrupt or polling mode.  Only the end user know if performance or cpu
>> usage is more important to their application.
> 
> Usually you can take a reasonable guess as to what would be a good point
> to start switching, typically for short enough transfers the overhead of
> setting up DMA, waiting for interrupts and tearing things down is very
> much larger than the cost of just doing PIO - a bunch of other drivers
> have pick a number logic of some kind, often things like FIFO sizes are
> a good key for where to look.  A lot of the time this is good enough,
> and it means that users have much better facilities for making tradeoffs
> if they have a range of transfer sizes available - it's not an either/or
> thing but based on some features of the individual message/transfer.
> 
> It is true that for people with heavy SPI traffic or otherwise very
> demanding requirements for a specific system and software stack
> additional tuning might produce better results, exposing some sysfs
> knobs to allow tuning of parameters at runtime would be helpful for them
> and I'd certainly be happy to see that added.
> 
Thanks for the explanation. I saw the spi-uniphier.c and spi-bcm2835.c 
doing the trick you mentioned(thanks Kursad for pointing out).  In our 
case, even the maximum fifo size usage(512bytes), the polling still have 
better performance than interrupt. The MTD test result included in this 
patch is based on maximum fifo usage. So there is no benefit to switch 
to interrupt based on transfer size.

Does the spi framework has any requirement on how much time that the 
driver's transfer_one function can spend?  I can see the polling 
function might take quite some time in busy loop if the clock is low, 
for example, at 100Hz(slowest clock this controller can go), it takes 
512x8/100Hz ~= 41ms to complete.  If this is something in concern,  I 
can do the interrupt switch based on a time limit value if interrupt is 
available.

--000000000000342f6e05f202a34b
Content-Type: application/pkcs7-signature; name="smime.p7s"
Content-Transfer-Encoding: base64
Content-Disposition: attachment; filename="smime.p7s"
Content-Description: S/MIME Cryptographic Signature

MIIQcAYJKoZIhvcNAQcCoIIQYTCCEF0CAQExDzANBglghkgBZQMEAgEFADALBgkqhkiG9w0BBwGg
gg3HMIIFDTCCA/WgAwIBAgIQeEqpED+lv77edQixNJMdADANBgkqhkiG9w0BAQsFADBMMSAwHgYD
VQQLExdHbG9iYWxTaWduIFJvb3QgQ0EgLSBSMzETMBEGA1UEChMKR2xvYmFsU2lnbjETMBEGA1UE
AxMKR2xvYmFsU2lnbjAeFw0yMDA5MTYwMDAwMDBaFw0yODA5MTYwMDAwMDBaMFsxCzAJBgNVBAYT
AkJFMRkwFwYDVQQKExBHbG9iYWxTaWduIG52LXNhMTEwLwYDVQQDEyhHbG9iYWxTaWduIEdDQyBS
MyBQZXJzb25hbFNpZ24gMiBDQSAyMDIwMIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEA
vbCmXCcsbZ/a0fRIQMBxp4gJnnyeneFYpEtNydrZZ+GeKSMdHiDgXD1UnRSIudKo+moQ6YlCOu4t
rVWO/EiXfYnK7zeop26ry1RpKtogB7/O115zultAz64ydQYLe+a1e/czkALg3sgTcOOcFZTXk38e
aqsXsipoX1vsNurqPtnC27TWsA7pk4uKXscFjkeUE8JZu9BDKaswZygxBOPBQBwrA5+20Wxlk6k1
e6EKaaNaNZUy30q3ArEf30ZDpXyfCtiXnupjSK8WU2cK4qsEtj09JS4+mhi0CTCrCnXAzum3tgcH
cHRg0prcSzzEUDQWoFxyuqwiwhHu3sPQNmFOMwIDAQABo4IB2jCCAdYwDgYDVR0PAQH/BAQDAgGG
MGAGA1UdJQRZMFcGCCsGAQUFBwMCBggrBgEFBQcDBAYKKwYBBAGCNxQCAgYKKwYBBAGCNwoDBAYJ
KwYBBAGCNxUGBgorBgEEAYI3CgMMBggrBgEFBQcDBwYIKwYBBQUHAxEwEgYDVR0TAQH/BAgwBgEB
/wIBADAdBgNVHQ4EFgQUljPR5lgXWzR1ioFWZNW+SN6hj88wHwYDVR0jBBgwFoAUj/BLf6guRSSu
TVD6Y5qL3uLdG7wwegYIKwYBBQUHAQEEbjBsMC0GCCsGAQUFBzABhiFodHRwOi8vb2NzcC5nbG9i
YWxzaWduLmNvbS9yb290cjMwOwYIKwYBBQUHMAKGL2h0dHA6Ly9zZWN1cmUuZ2xvYmFsc2lnbi5j
b20vY2FjZXJ0L3Jvb3QtcjMuY3J0MDYGA1UdHwQvMC0wK6ApoCeGJWh0dHA6Ly9jcmwuZ2xvYmFs
c2lnbi5jb20vcm9vdC1yMy5jcmwwWgYDVR0gBFMwUTALBgkrBgEEAaAyASgwQgYKKwYBBAGgMgEo
CjA0MDIGCCsGAQUFBwIBFiZodHRwczovL3d3dy5nbG9iYWxzaWduLmNvbS9yZXBvc2l0b3J5LzAN
BgkqhkiG9w0BAQsFAAOCAQEAdAXk/XCnDeAOd9nNEUvWPxblOQ/5o/q6OIeTYvoEvUUi2qHUOtbf
jBGdTptFsXXe4RgjVF9b6DuizgYfy+cILmvi5hfk3Iq8MAZsgtW+A/otQsJvK2wRatLE61RbzkX8
9/OXEZ1zT7t/q2RiJqzpvV8NChxIj+P7WTtepPm9AIj0Keue+gS2qvzAZAY34ZZeRHgA7g5O4TPJ
/oTd+4rgiU++wLDlcZYd/slFkaT3xg4qWDepEMjT4T1qFOQIL+ijUArYS4owpPg9NISTKa1qqKWJ
jFoyms0d0GwOniIIbBvhI2MJ7BSY9MYtWVT5jJO3tsVHwj4cp92CSFuGwunFMzCCA18wggJHoAMC
AQICCwQAAAAAASFYUwiiMA0GCSqGSIb3DQEBCwUAMEwxIDAeBgNVBAsTF0dsb2JhbFNpZ24gUm9v
dCBDQSAtIFIzMRMwEQYDVQQKEwpHbG9iYWxTaWduMRMwEQYDVQQDEwpHbG9iYWxTaWduMB4XDTA5
MDMxODEwMDAwMFoXDTI5MDMxODEwMDAwMFowTDEgMB4GA1UECxMXR2xvYmFsU2lnbiBSb290IENB
IC0gUjMxEzARBgNVBAoTCkdsb2JhbFNpZ24xEzARBgNVBAMTCkdsb2JhbFNpZ24wggEiMA0GCSqG
SIb3DQEBAQUAA4IBDwAwggEKAoIBAQDMJXaQeQZ4Ihb1wIO2hMoonv0FdhHFrYhy/EYCQ8eyip0E
XyTLLkvhYIJG4VKrDIFHcGzdZNHr9SyjD4I9DCuul9e2FIYQebs7E4B3jAjhSdJqYi8fXvqWaN+J
J5U4nwbXPsnLJlkNc96wyOkmDoMVxu9bi9IEYMpJpij2aTv2y8gokeWdimFXN6x0FNx04Druci8u
nPvQu7/1PQDhBjPogiuuU6Y6FnOM3UEOIDrAtKeh6bJPkC4yYOlXy7kEkmho5TgmYHWyn3f/kRTv
riBJ/K1AFUjRAjFhGV64l++td7dkmnq/X8ET75ti+w1s4FRpFqkD2m7pg5NxdsZphYIXAgMBAAGj
QjBAMA4GA1UdDwEB/wQEAwIBBjAPBgNVHRMBAf8EBTADAQH/MB0GA1UdDgQWBBSP8Et/qC5FJK5N
UPpjmove4t0bvDANBgkqhkiG9w0BAQsFAAOCAQEAS0DbwFCq/sgM7/eWVEVJu5YACUGssxOGhigH
M8pr5nS5ugAtrqQK0/Xx8Q+Kv3NnSoPHRHt44K9ubG8DKY4zOUXDjuS5V2yq/BKW7FPGLeQkbLmU
Y/vcU2hnVj6DuM81IcPJaP7O2sJTqsyQiunwXUaMld16WCgaLx3ezQA3QY/tRG3XUyiXfvNnBB4V
14qWtNPeTCekTBtzc3b0F5nCH3oO4y0IrQocLP88q1UOD5F+NuvDV0m+4S4tfGCLw0FREyOdzvcy
a5QBqJnnLDMfOjsl0oZAzjsshnjJYS8Uuu7bVW/fhO4FCU29KNhyztNiUGUe65KXgzHZs7XKR1g/
XzCCBU8wggQ3oAMCAQICDDG6HZcbcVdEvVYk4TANBgkqhkiG9w0BAQsFADBbMQswCQYDVQQGEwJC
RTEZMBcGA1UEChMQR2xvYmFsU2lnbiBudi1zYTExMC8GA1UEAxMoR2xvYmFsU2lnbiBHQ0MgUjMg
UGVyc29uYWxTaWduIDIgQ0EgMjAyMDAeFw0yMjA5MTAxMTMxNDVaFw0yNTA5MTAxMTMxNDVaMIGQ
MQswCQYDVQQGEwJJTjESMBAGA1UECBMJS2FybmF0YWthMRIwEAYDVQQHEwlCYW5nYWxvcmUxFjAU
BgNVBAoTDUJyb2FkY29tIEluYy4xFjAUBgNVBAMTDVdpbGxpYW0gWmhhbmcxKTAnBgkqhkiG9w0B
CQEWGndpbGxpYW0uemhhbmdAYnJvYWRjb20uY29tMIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIB
CgKCAQEAyKF+RmY29Wvfmfe3L8J4rZNmBIvRmrWKI5td5L0vlpPMCEzUkVhBdL2N9cDP0rPScvWL
CX/9cI1a2BUy/6/ZT5j9PhcUn6A3kwKFGukLY2itfKaDrP3ANVJGhBXPVJ6sx55GF41PkiL2EMnY
7LJGNpl9WHYrw8VqtRediPyXq8M6ZWGPZWxygsE6y1pOkEk9qLpvXTb2Epxk2JWcQFZQCDWVULue
YDZuuBJwnyCzevMoPtVYPharioL5H3BRnQi8YoTXH7/uRo33dewYFm474yFjwwnt82TFtveVZkVq
6h4WIQ4wTcwFfET8zMkELnGzS5SHCl8sPD+lNxxJ1JDZYwIDAQABo4IB2zCCAdcwDgYDVR0PAQH/
BAQDAgWgMIGjBggrBgEFBQcBAQSBljCBkzBOBggrBgEFBQcwAoZCaHR0cDovL3NlY3VyZS5nbG9i
YWxzaWduLmNvbS9jYWNlcnQvZ3NnY2NyM3BlcnNvbmFsc2lnbjJjYTIwMjAuY3J0MEEGCCsGAQUF
BzABhjVodHRwOi8vb2NzcC5nbG9iYWxzaWduLmNvbS9nc2djY3IzcGVyc29uYWxzaWduMmNhMjAy
MDBNBgNVHSAERjBEMEIGCisGAQQBoDIBKAowNDAyBggrBgEFBQcCARYmaHR0cHM6Ly93d3cuZ2xv
YmFsc2lnbi5jb20vcmVwb3NpdG9yeS8wCQYDVR0TBAIwADBJBgNVHR8EQjBAMD6gPKA6hjhodHRw
Oi8vY3JsLmdsb2JhbHNpZ24uY29tL2dzZ2NjcjNwZXJzb25hbHNpZ24yY2EyMDIwLmNybDAlBgNV
HREEHjAcgRp3aWxsaWFtLnpoYW5nQGJyb2FkY29tLmNvbTATBgNVHSUEDDAKBggrBgEFBQcDBDAf
BgNVHSMEGDAWgBSWM9HmWBdbNHWKgVZk1b5I3qGPzzAdBgNVHQ4EFgQUq65GzwZxydFHjjYEU/9h
xHhPWlwwDQYJKoZIhvcNAQELBQADggEBAA2hGG3JPAdGPH0ZdohGUCIVjKz+U+EFuIDbS6A/5jqX
VhYAxZlzj7tSjUIM7G7IhyfqPC46GKJ/4x+Amz1Z6YxNGy71L68kYD6hIbBcA5AM42QBUufly6Oa
/ppSz3WoflVyFFQ5YXniZ+eU+2/cdnYZg4aVUnFjimOF5o3NfMLzOkhQNxbaDjFUfUYD8hKmU6v4
0vUBj8KZ9Gi1LIagLKUREn8jku0lcLsRbnJ5Ey5ScajC/FESPyYWasOW8j8/1EoJksmhbYGKNS6C
urb/KlmDGfVrIRYDbL0ckhGQIP5c6L+kSQZ2sHnQK0e0WgIaZYxaPYeY5u0GLCOze+3vyRMxggJt
MIICaQIBATBrMFsxCzAJBgNVBAYTAkJFMRkwFwYDVQQKExBHbG9iYWxTaWduIG52LXNhMTEwLwYD
VQQDEyhHbG9iYWxTaWduIEdDQyBSMyBQZXJzb25hbFNpZ24gMiBDQSAyMDIwAgwxuh2XG3FXRL1W
JOEwDQYJYIZIAWUDBAIBBQCggdQwLwYJKoZIhvcNAQkEMSIEIN8lIdnOi8ZOM95L8yp3bo3fofzT
ZY638Ip31W1AZylyMBgGCSqGSIb3DQEJAzELBgkqhkiG9w0BBwEwHAYJKoZIhvcNAQkFMQ8XDTIz
MDExMTIwMTM0NVowaQYJKoZIhvcNAQkPMVwwWjALBglghkgBZQMEASowCwYJYIZIAWUDBAEWMAsG
CWCGSAFlAwQBAjAKBggqhkiG9w0DBzALBgkqhkiG9w0BAQowCwYJKoZIhvcNAQEHMAsGCWCGSAFl
AwQCATANBgkqhkiG9w0BAQEFAASCAQBiJGKb56Hcg9H2fKVlw9sJNfauEUZq4xr6IAw2LfN76sag
mB3pios2Tftk188pvaG7VJnLwkYlN5iaiOlASJORbXb6kD7NwPuVDJNlu6E/BnUV63p5EbEcOxej
LCq5cu9GAbKNcjVoXnHAo/np9yCwLYCKZqjyfydtkMLeFt3oCG+b3w0+TYq7wzEmmq1DNMAvnvnc
NguS0DoVZ4R79wBm0UEaEemRo8u8tbf/qR//C+PosJkLdBaHhe45mBSrcTi8L2jxJf6s/9IaJz/p
DJPcB3LaZEcLGRcPUGk0qGGF28yRGFLesXXqdZju98z+roWx4iCa3bm9202FviLUiIWu
--000000000000342f6e05f202a34b--