Towards the bottom of amud a - Rava says ayn keva L'sheyna. My first read of the gemara was that Rava was contending against R ashi who said that the reason why you cant take a nap outside of succa was a gezira. Rava responds that the reason is more fundamental. However, Rashi seems to be saying that Rava is answering abyay's question of shouldn't tefillin also have a gezira. Rashi ;s first words on DH Rava indicate that for rashi - Rava is also addressing the question of tefillin and not just sukka. Rashi seems to make up that Rava holds that there is no chashah for shema yiradem.
How does Rashi know this? DO I have a correct read of Rashi?
Y, NY
I think you were basically on the right lines with your first reading. However we have to add on an important point - that there is another question in the Sugya: Does Rava agree with Rabah bar bar Chanah in the name of Rabbi Yochanan, that one is only allowed to sleep with one's Tefilin on if one puts one's head between one's knees?
Rashi indeed maintains that Rava argues with R. Yochanan as well as Rav Ashi. Therefore Rashi writes that there is no Chashash of Shema Yiradem, and consequently Rashi adds immediately "therefore in Tefilin this is permitted" i.e. Rashi means that temporary sleep is permitted in Tefilin with no conditions. Because Rashi stated that there is absolutely no problem with Tefilin he then has to stress the contrast with Sukah: even a minimal sleep is considered as sleeping outside the Sukah.
You asked how Rashi knows that there is no Chashash of Shema Yiradem? Well, firstly it might be that the Gemara reads more smoothly if one says that Rava is a new opinion, which rejects the previous opinions in the Gemara. [This is hinted at by the Ba'al ha'Ma'or (p.12a in Rif pages) who writes, "He does not need to place his head between his knees, because we rely on Rava, who is the final opinion, and does not agree with the suggestions of the earlier Amoraim - that he asked someone else to wake him up, or that he placed his head between his knees". The Ba'al ha'Ma'or's statement that Rava is the final opinion, also helps to answer your question, because there is a rule in the Gemara that the Halachah usually follows the opinion of the latest Amora.]
There is another proof that Rava disagrees with R. Yochanan (see the Sfas Emes and Otzar Mefarshei ha'Talmud). This is from the Beraisa (below 26b) which cites a dispute between Rebbi Nassan and Rebbi Yosi. R. Nassan prohibits wearing Tefilin when sleeping at night and R. Yosi prohibits even during the daytime for young men together with their wives. If the Halachah would have followed R. Yochanan that one must always place one's head between the knees, then why should R. Nassan be stringent at night? Even if one should answer this by saying that the head between the knees is not sufficient at night because when he wakes up from his temporary sleep he might forget and fall into a heavy sleep for the entire night, nevertheless the head between the knees should certainly have been sufficient according to R. Yosi, during the day at least, to prevent misuse and Bizayon. From the fact that R. Yosi was stringent one can prove that the Halachah does not follow R. Yochanan, so the practice
of the head between the knees is never used. This is why both R. Nassan and R. Yosi were stringent.
[Some Rishonim maintain that Rava agrees with R. Yochanan - see Ritva, Rambam and others - but Rashi writes that there is no Chashash of Shema Yiradem, as you pointed out, which suggests that there is no need at all to place the head between the knees.]
D. Bloom